diff --git a/blockly_uncompressed.js b/blockly_uncompressed.js
deleted file mode 100644
index 804f97f0b..000000000
--- a/blockly_uncompressed.js
+++ /dev/null
@@ -1,58 +0,0 @@
-/**
- * @license
- * Copyright 2020 Google LLC
- * SPDX-License-Identifier: Apache-2.0
- */
-
-/**
- * @fileoverview Bootstrap code to load Blockly in uncompiled mode.
- */
-'use strict';
-
-
-/* eslint-disable no-var */
-
-/**
- * Blockly uncompiled-mode startup code.  If running in a browser
- * loads closure/goog/base.js and build/deps.js, then (in any case)
- * requires Blockly.requires.
- */
-(function(globalThis) {
-  /* eslint-disable-next-line no-undef */
-  var IS_NODE_JS = !!(typeof module !== 'undefined' && module.exports);
-
-  if (IS_NODE_JS) {
-    // Load Blockly.
-    goog.require('Blockly.requires');
-    /* eslint-disable no-undef */
-    module.exports = Blockly;
-  } else {
-    var BLOCKLY_DIR = '';
-    // Find name of current directory.
-    var scripts = document.getElementsByTagName('script');
-    var re = /(.+)[\\/]blockly_(?:.*)uncompressed\.js$/;
-    for (var script, i = 0; (script = scripts[i]); i++) {
-      var match = re.exec(script.src);
-      if (match) {
-        BLOCKLY_DIR = match[1];
-        break;
-      }
-    }
-    if (!BLOCKLY_DIR) {
-      alert('Could not detect Blockly\'s directory name.');
-    }
-    // Disable loading of closure/goog/deps.js (which doesn't exist).
-    globalThis.CLOSURE_NO_DEPS = true;
-    // Load Closure Library base.js (the only part of the libary we use,
-    // mainly for goog.require / goog.provide / goog.module).
-    document.write(
-        '<script src="' + BLOCKLY_DIR + '/closure/goog/base.js"></script>');
-    // Load dependency graph info from build/deps.js.  To update
-    // deps.js, run `npm run build:deps`.
-    document.write(
-        '<script src="' + BLOCKLY_DIR + '/build/deps.js"></script>');
-    // Load the rest of Blockly.
-    document.write('<script>goog.require(\'Blockly\');</script>');
-  }
-  /* eslint-disable-next-line no-invalid-this */
-})(this);
diff --git a/scripts/gulpfiles/build_tasks.js b/scripts/gulpfiles/build_tasks.js
index 9b6114bc5..1da8c3325 100644
--- a/scripts/gulpfiles/build_tasks.js
+++ b/scripts/gulpfiles/build_tasks.js
@@ -262,7 +262,7 @@ function buildJavaScript(done) {
 
 /**
  * This task updates DEPS_FILE (deps.js), used by
- * blockly_uncompressed.js when loading Blockly in uncompiled mode.
+ * bootstrap.js when loading Blockly in uncompiled mode.
  *
  * Also updates TEST_DEPS_FILE (deps.mocha.js), used by the mocha test
  * suite.
@@ -673,7 +673,6 @@ function buildAdvancedCompilationTest() {
  *     php_compressed.js
  *     lua_compressed.js
  *     dart_compressed.js
- *     blockly_uncompressed.js
  *     msg/json/*.js
  *     test/deps*.js
  */
diff --git a/scripts/gulpfiles/config.js b/scripts/gulpfiles/config.js
index 9080866fa..02df80291 100644
--- a/scripts/gulpfiles/config.js
+++ b/scripts/gulpfiles/config.js
@@ -19,17 +19,16 @@ var path = require('path');
 // - tests/scripts/compile_typings.sh
 // - tests/scripts/check_metadata.sh
 // - tests/scripts/update_metadata.sh
-// - blockly_uncompressed.js (for location of deps.js)
-// - tests/playgrounds/prepare.js (for location of deps.js)
+// - tests/bootstrap.js (for location of deps.js)
 // - tests/mocha/index.html (for location of deps.mocha.js)
 
 // Directory to write compiled output to.
 exports.BUILD_DIR = 'build';
 
-// Dependencies file (for blockly_uncompressed.js):
+// Dependencies file (used by bootstrap.js in uncompiled mode):
 exports.DEPS_FILE = path.join(exports.BUILD_DIR, 'deps.js');
 
-// Dependencies file (for blockly_uncompressed.js):
+// Mocha test dependencies file (used by tests/mocha/index.html):
 exports.TEST_DEPS_FILE = path.join(exports.BUILD_DIR, 'deps.mocha.js');
 
 // Directory to write typings output to.
diff --git a/tests/bootstrap.js b/tests/bootstrap.js
new file mode 100644
index 000000000..45f64bb19
--- /dev/null
+++ b/tests/bootstrap.js
@@ -0,0 +1,246 @@
+/**
+ * @license
+ * Copyright 2021 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+/**
+ * @fileoverview Bootstrap code to load Blockly, typically in
+ *     uncompressed mode.
+ *
+ *     Load this file in a <script> tag in a web page to use the
+ *     Closure Library debug module loader to load Blockly in
+ *     uncompressed mode.
+ *
+ *     You must use a <script src=> tag to load this script first
+ *     (after setting BLOCKLY_BOOTSTRAP_OPTIONS in a preceeding
+ *     <script> tag, if desired - see below), then import
+ *     bootstrap_done.mjs in a <script type=module> to wait for
+ *     bootstrapping to finish.
+ *
+ *     See tests/playground.html for example usage.
+ *
+ *     Exception: for speed and compatibility reasons, if this is
+ *     script is loaded in Internet Explorer or from a host other than
+ *     localhost, blockly_compressed.js (et al.) will be loaded
+ *     instead.  IE 11 doesn't understand modern JavaScript, and
+ *     because of the sequential, non-parallel module loading carried
+ *     out by thedeubg module loader can be painfully tedious over a
+ *     slow network connection.  (This can be overridden by the page
+ *     if desired.)
+ *
+ *     (Note also that this file eschews certain modern JS constructs,
+ *     like template literals, for compatibility with IE 11.)
+ *
+ *     The bootstrap code will consult a BLOCKLY_BOOTSTRAP_OPTIONS
+ *     global variable to determine what to load.  This must be set
+ *     before loading this script.  See documentation inline below.
+ *
+ */
+'use strict';
+
+(function() {
+  // Values used to compute default bootstrap options.
+  const isIe = navigator.userAgent.indexOf('MSIE') !== -1 ||
+      navigator.appVersion.indexOf('Trident/') > -1;
+  const localhosts = ['localhost', '127.0.0.1', '[::1]'];
+  const isLocalhost = localhosts.includes(location.hostname);
+  const isFileUrl = location.origin === 'file://';
+
+  // Default bootstrap options.  These can be overridden by setting
+  // the same property on BLOCKLY_BOOTSTRAP_OPTIONS.
+  const options = {
+    // Decide whether to use compressed mode or not.  Please see issue
+    // #5557 for more information.
+    loadCompressed: isIe || !(isLocalhost || isFileUrl),
+
+    // URL of the blockly repository.  This is needed for a few reasons:
+    //
+    // - We need an absolute path instead of relative path because the
+    //   advanced_playground and the regular playground are in
+    //   different folders.
+    // - We need to get the root directory for blockly because it is
+    //   different for github.io, appspot and local.
+    //
+    // Default value will work so long as top-level page is loaded
+    // from somewhere in tests/.
+    root: window.location.href.replace(/\/tests\/.*$/, '/'),
+
+    // List of deps files to load.  Paths relative to root.
+    depsFiles: [
+      'build/deps.js',
+    ],
+    
+    // List of goog.modules to goog.require.
+    requires: [
+      'Blockly',
+      'Blockly.libraryBlocks',
+      'Blockly.Dart.all',
+      'Blockly.JavaScript.all',
+      'Blockly.Lua.all',
+      'Blockly.PHP.all',
+      'Blockly.Python.all',
+    ],
+
+    // List of scripts to load in compressed mode, instead of
+    // requires.  Paths relative to root.
+    compressedScripts: [
+      'blockly_compressed.js',
+      'blocks_compressed.js',
+      'dart_compressed.js',
+      'javascript_compressed.js',
+      'lua_compressed.js',
+      'php_compressed.js',
+      'python_compressed.js',
+    ],
+
+    // Additional scripts to be loaded after Blockly is loaded,
+    // whether Blockly is loaded from compressed or uncompressed.
+    // Paths relative to root.
+    additionalScripts: [
+      'msg/messages.js',
+    ],
+  };
+  if (typeof window.BLOCKLY_BOOTSTRAP_OPTIONS === 'object') {
+    Object.assign(options, window.BLOCKLY_BOOTSTRAP_OPTIONS);
+  }
+
+  /* eslint-disable-next-line no-undef */
+  if (typeof module === 'object' && typeof module.exports === 'object') {
+    // Running in node.js.  Maybe we wish to support this.
+    // blockly_uncompressed formerly did, though it appears that the
+    // code had not been working for some time (at least since PR
+    // #5718 back in December 2021.  For now just throw an error.
+    throw new Error('Bootstrapping in node.js not implemented.');
+  }
+
+  // Create a global variable to remember some state that will be
+  // needed by later scripts.
+  window.bootstrapInfo = {
+    /** boolean */ compressed: options.loadCompressed,
+    /** ?Array<string> */ requires: null,
+    /** ?Promise */ done: null,
+  };
+
+  if (!options.loadCompressed) {
+    // We can load Blockly in uncompressed mode.  Note that this section
+    // needs to parse in IE11 (mostly ES5.1, but allowing e.g. const),
+    // but it does not need to be _executable_ in IE11 - it is safe to
+    // use ES6 builtins.
+
+    // Disable loading of closure/goog/deps.js (which doesn't exist).
+    window.CLOSURE_NO_DEPS = true;
+    // Load the Closure Library's base.js (the only part of the
+    // libary we use, mainly for goog.require / goog.provide /
+    // goog.module).
+    document.write(
+        '<script src="' + options.root + '/closure/goog/base.js"></script>');
+
+    // Prevent spurious transpilation warnings.
+    document.write('<script>goog.TRANSPILE = "never";</script>');
+
+    // Load dependency graph info from the specified deps files -
+    // typically just build/deps.js.  To update deps after changing
+    // any module's goog.requires / imports, run `npm run build:deps`.
+    for (let i = 0; i < options.depsFiles.length; i++) {
+      document.write(
+          '<script src="' + options.root + options.depsFiles[i] + '">' +
+              '</script>');
+    }
+
+    // Record require targets for bootstrap_helper.js.
+    window.bootstrapInfo.requires = options.requires;
+
+    // Assemble a list of module targets to bootstrap.
+    //
+    // The first group of targets are those listed in
+    // options.requires.
+    //
+    // The next target is a fake one that will load
+    // bootstrap_helper.js.  We generate a call to goog.addDependency
+    // to tell the debug module loader that it can be loaded via a
+    // fake module name, and that it depends on all the targets in the
+    // first group (and indeed bootstrap_helper.js will make a call to
+    // goog.require for each one).
+    //
+    // We then create another target for each of
+    // options.additionalScripts, again generating calls to
+    // goog.addDependency for each one making it dependent on the
+    // previous one.
+    let requires = options.requires.slice();
+    const scripts =
+        ['tests/bootstrap_helper.js'].concat(options.additionalScripts);
+    const scriptDeps = [];
+    for (let script, i = 0; script = scripts[i]; i++) {
+      const fakeModuleName = 'script.' + script.replace(/[./]/g, "-");
+      scriptDeps.push('  goog.addDependency(' +
+          quote('../../' + script) + ', [' + quote(fakeModuleName) +
+          '], [' + requires.map(quote).join() + "], {'lang': 'es6'});\n");
+      requires = [fakeModuleName];
+    }
+
+    // Finally, write out a script containing the generated
+    // goog.addDependency calls and a call to goog.bootstrap
+    // requesting the loading of the final target, which will cause
+    // all the previous ones to be loaded recursively.  Wrap this in a
+    // promise and save it so it can be awaited in bootstrap_done.mjs.
+    document.write(
+        '<script>\n' + scriptDeps.join('') +
+        '  window.bootstrapInfo.done = new Promise((resolve, reject) => {\n' +
+        '    goog.bootstrap([' + requires.map(quote).join() + '], resolve);\n' +
+        '  });\n' +
+        '</script>\n');
+  } else {
+    // We need to load Blockly in compressed mode.  Load
+    // blockly_compressed.js et al. using <script> tags.
+    const scripts =
+        options.compressedScripts.concat(options.additionalScripts);
+    for (let i = 0; i < scripts.length; i++) {
+      document.write(
+          '<script src="' + options.root + scripts[i] + '"></script>');
+    }
+  }
+
+  return;  // All done.  Only helper functions after this point.
+
+  /**
+   * Convert a string into a string literal.  Strictly speaking we
+   * only need to escape backslash, \r, \n, \u2028 (line separator),
+   * \u2029 (paragraph separator) and whichever quote character we're
+   * using, but for simplicity we escape all the control characters.
+   *
+   * Based on https://github.com/google/CodeCity/blob/master/server/code.js
+   *
+   * @param {string} str The string to convert.
+   * @return {string} The value s as a eval-able string literal.
+   */
+  function quote(str) {
+    /* eslint-disable no-control-regex, no-multi-spaces */
+    /** Regexp for characters to be escaped in a single-quoted string. */
+    const singleRE = /[\x00-\x1f\\\u2028\u2029']/g;
+
+    /** Map of control character replacements. */
+    const replacements = {
+      '\x00': '\\0',   '\x01': '\\x01', '\x02': '\\x02', '\x03': '\\x03',
+      '\x04': '\\x04', '\x05': '\\x05', '\x06': '\\x06', '\x07': '\\x07',
+      '\x08': '\\b',   '\x09': '\\t',   '\x0a': '\\n',   '\x0b': '\\v',
+      '\x0c': '\\f',   '\x0d': '\\r',   '\x0e': '\\x0e', '\x0f': '\\x0f',
+      '"': '\\"', "'": "\\'", '\\': '\\\\',
+      '\u2028': '\\u2028', '\u2029': '\\u2029',
+    };
+    /* eslint-enable no-control-regex, no-multi-spaces */
+
+    /**
+     * Replacer function.
+     * @param {string} c Single UTF-16 code unit ("character") string to
+     *     be replaced.
+     * @return {string} Multi-character string containing escaped
+     *     representation of c.
+     */
+    function replace(c) {
+      return replacements[c];
+    }
+
+    return "'" + str.replace(singleRE, replace) + "'";
+  }
+})();
diff --git a/tests/playgrounds/blockly.mjs b/tests/bootstrap_done.mjs
similarity index 57%
rename from tests/playgrounds/blockly.mjs
rename to tests/bootstrap_done.mjs
index cb836f539..d6d0c9b31 100644
--- a/tests/playgrounds/blockly.mjs
+++ b/tests/bootstrap_done.mjs
@@ -20,20 +20,20 @@
  *
  *     See tests/playground.html for example usage.
  */
-
-let Blockly;
-
-if (window.BlocklyLoader) {
-  // Uncompiled mode.  Use top-level await
-  // (https://v8.dev/features/top-level-await) to block loading of
-  // this module until goog.bootstrap()ping of Blockly is finished.
-  await window.BlocklyLoader;
-  Blockly = globalThis.Blockly;
-} else if (window.Blockly) {
-  // Compiled mode.  Retrieve the pre-installed Blockly global.
-  Blockly = globalThis.Blockly;
-} else {
-  throw new Error('neither window.Blockly nor window.BlocklyLoader found');
+if (!window.bootstrapInfo) {
+  throw new Error('window.bootstrapInfo not found.  ' +
+      'Make sure to load bootstrap.js before importing bootstrap_done.mjs.');
 }
 
-export default Blockly;
+if (window.bootstrapInfo.compressed) {
+  // Compiled mode.  Nothing more to do.
+} else {
+  // Uncompressed mode.  Use top-level await
+  // (https://v8.dev/features/top-level-await) to block loading of
+  // this module until goog.bootstrap()ping of Blockly is finished.
+  await window.bootstrapInfo.done;
+  // Note that this module previously did an export default of the
+  // value returned by the bootstrapInfo.done promise.  This was
+  // changed in PR #5995 because library blocks and generators cannot
+  // be accessed via that the core/blockly.js exports object.
+}
diff --git a/tests/bootstrap_helper.js b/tests/bootstrap_helper.js
new file mode 100644
index 000000000..ba61ff2ea
--- /dev/null
+++ b/tests/bootstrap_helper.js
@@ -0,0 +1,20 @@
+/**
+ * @license
+ * Copyright 2022 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+/**
+ * @fileoverview Helper script for bootstrap.js
+ *
+ * This is loaded, via goog.bootstrap(), after the other top-level
+ * Blockly modules.  It simply calls goog.require() for each of them,
+ * to force the debug module loader to finish loading them before any
+ * non-module scripts (like msg/messages.js) that might have
+ * undeclared dependencies on them.
+ */
+
+/* eslint-disable-next-line no-undef */
+for (const require of window.bootstrapInfo.requires) {
+  goog.require(require);
+}
diff --git a/tests/generators/index.html b/tests/generators/index.html
index 9e37f05b6..69badeed1 100644
--- a/tests/generators/index.html
+++ b/tests/generators/index.html
@@ -3,41 +3,36 @@
 <head>
 <meta charset="utf-8">
 <title>Blockly Generator Tests</title>
-<script src="../../blockly_uncompressed.js"></script>
-<script src="../../msg/messages.js"></script>
-
 <script>
-// Require optional modules needed by tests.
-goog.require('Blockly.Dart.all');
-goog.require('Blockly.JavaScript.all');
-goog.require('Blockly.Lua.all');
-goog.require('Blockly.PHP.all');
-goog.require('Blockly.Python.all');
-goog.require('Blockly.libraryBlocks');
+  var BLOCKLY_BOOTSTRAP_OPTIONS = {
+    additionalScripts: [
+      'msg/messages.js',
+      'tests/generators/unittest_javascript.js',
+      'tests/generators/unittest_python.js',
+      'tests/generators/unittest_php.js',
+      'tests/generators/unittest_lua.js',
+      'tests/generators/unittest_dart.js',
+      'tests/generators/unittest.js',
+    ],
+  }
 </script>
-<script src="unittest_javascript.js"></script>
-<script src="unittest_python.js"></script>
-<script src="unittest_php.js"></script>
-<script src="unittest_lua.js"></script>
-<script src="unittest_dart.js"></script>
-<script src="unittest.js"></script>
+<script src="../bootstrap.js"></script>
 <script>
-'use strict';
-
 var demoWorkspace = null;
 var outputCode = '';
 
 function start() {
-  demoWorkspace = Blockly.inject('blocklyDiv',
-      {grid:
-         {spacing: 25,
-          length: 3,
-          colour: '#ccc',
-          snap: true},
-       media: '../../media/',
-       toolbox: document.getElementById('toolbox'),
-       zoom: {controls: true, wheel: true}
-       });
+  demoWorkspace = Blockly.inject('blocklyDiv', {
+    grid: {
+      spacing: 25,
+      length: 3,
+      colour: '#ccc',
+      snap: true
+    },
+    media: '../../media/',
+    toolbox: document.getElementById('toolbox'),
+    zoom: {controls: true, wheel: true}
+  });
   changeIndex();
 }
 
@@ -210,6 +205,12 @@ function changeIndex() {
   demoWorkspace.getToolbox().flyout_.workspace_.options.oneBasedIndex = oneBasedIndex;
 }
 </script>
+<script type=module>
+// Wait for Blockly to finish loading before running tests.
+import '../bootstrap_done.mjs';
+
+start();
+</script>
 
 <style>
 html, body {
@@ -240,7 +241,7 @@ h1 {
 }
 </style>
 </head>
-<body onload="start()">
+<body>
 
   <div id="blocklyDiv"></div>
 
diff --git a/tests/mocha/.mocharc.js b/tests/mocha/.mocharc.js
index 3c156db3e..0928b25d2 100644
--- a/tests/mocha/.mocharc.js
+++ b/tests/mocha/.mocharc.js
@@ -2,6 +2,5 @@
 
 module.exports = {
   ui: 'tdd',
-  file: '../blockly_uncompressed.js',
   reporter: 'landing'
 };
diff --git a/tests/mocha/index.html b/tests/mocha/index.html
index c3caf1469..10fe5ed45 100644
--- a/tests/mocha/index.html
+++ b/tests/mocha/index.html
@@ -5,9 +5,6 @@
     <title>Mocha Tests for Blockly</title>
 
     <link href="../../node_modules/mocha/mocha.css" rel="stylesheet" />
-    <script src="../../blockly_uncompressed.js"></script>
-    <script src="../../build/deps.mocha.js"></script>
-    <script src="../../msg/messages.js"></script>
   </head>
   <style>
     #blocklyDiv {
@@ -21,6 +18,9 @@
     <div id="mocha"></div>
     <div id="failureCount" style="display:none" tests_failed="unset"></div>
     <div id="failureMessages" style="display:none"></div>
+    <!-- Load mocha et al. before bootstrapping so that we can safely
+	 goog.require() the test modules that make calls to (e.g.)
+	 suite() at the top level. -->
     <script src="../../node_modules/chai/chai.js"></script>
     <script src="../../node_modules/mocha/mocha.js"></script>
     <script src="../../node_modules/sinon/pkg/sinon.js"></script>
@@ -29,92 +29,95 @@
         ui: 'tdd',
         failZero: true,
       });
-    </script>
 
-    <script>
-      // Require optional modules needed by tests.
-      goog.require('Blockly.Dart');
-      goog.require('Blockly.Dart.texts');
-      goog.require('Blockly.JavaScript');
-      goog.require('Blockly.JavaScript.texts');
-      goog.require('Blockly.Lua');
-      goog.require('Blockly.Lua.texts');
-      goog.require('Blockly.PHP');
-      goog.require('Blockly.PHP.texts');
-      goog.require('Blockly.Python');
-      goog.require('Blockly.Python.texts');
-      goog.require('Blockly.libraryBlocks.colour');
-      goog.require('Blockly.libraryBlocks.logic');
-      goog.require('Blockly.libraryBlocks.lists');
-      goog.require('Blockly.libraryBlocks.loops');
-      goog.require('Blockly.libraryBlocks.math');
-      goog.require('Blockly.libraryBlocks.procedures');
-      goog.require('Blockly.libraryBlocks.texts');
-      goog.require('Blockly.libraryBlocks.variables');
-      goog.require('Blockly.libraryBlocks.variablesDynamic');
+      var BLOCKLY_BOOTSTRAP_OPTIONS = {
+	loadCompressed: false,
+	depsFiles: ['build/deps.js', 'build/deps.mocha.js'],
+	requires: [
+	  // Blockly modules needed by tests.
+	  'Blockly',
+	  'Blockly.libraryBlocks',
+	  'Blockly.Dart',
+	  'Blockly.Dart.texts',
+	  'Blockly.JavaScript',
+	  'Blockly.JavaScript.texts',
+	  'Blockly.Lua',
+	  'Blockly.Lua.texts',
+	  'Blockly.PHP',
+	  'Blockly.PHP.texts',
+	  'Blockly.Python',
+	  'Blockly.Python.texts',
 
-      // Run tests.
-      goog.require('Blockly.test.astNode');
-      goog.require('Blockly.test.blockChangeEvent');
-      goog.require('Blockly.test.blockCreateEvent');
-      goog.require('Blockly.test.blockJson');
-      goog.require('Blockly.test.blocks');
-      goog.require('Blockly.test.comments');
-      goog.require('Blockly.test.commentDeserialization');
-      goog.require('Blockly.test.connectionChecker');
-      goog.require('Blockly.test.connectionDb');
-      goog.require('Blockly.test.connection');
-      goog.require('Blockly.test.contextMenuItem');
-      goog.require('Blockly.test.cursor');
-      goog.require('Blockly.test.dropdown');
-      goog.require('Blockly.test.event');
-      goog.require('Blockly.test.extensions');
-      goog.require('Blockly.test.fieldAngle');
-      goog.require('Blockly.test.fieldCheckbox');
-      goog.require('Blockly.test.fieldColour');
-      goog.require('Blockly.test.fieldDropdown');
-      goog.require('Blockly.test.fieldImage');
-      goog.require('Blockly.test.fieldLabelSerialization');
-      goog.require('Blockly.test.fieldLabel');
-      goog.require('Blockly.test.fieldMultiline');
-      goog.require('Blockly.test.fieldNumber');
-      goog.require('Blockly.test.fieldRegistry');
-      goog.require('Blockly.test.fieldTest');
-      goog.require('Blockly.test.fieldTextInput');
-      goog.require('Blockly.test.fieldVariable');
-      goog.require('Blockly.test.flyout');
-      goog.require('Blockly.test.generator');
-      goog.require('Blockly.test.gesture');
-      goog.require('Blockly.test.input');
-      goog.require('Blockly.test.insertionMarker');
-      goog.require('Blockly.test.jsoDeserialization');
-      goog.require('Blockly.test.jsoSerialization');
-      goog.require('Blockly.test.json');
-      goog.require('Blockly.test.keydown');
-      goog.require('Blockly.test.lists');
-      goog.require('Blockly.test.logicTernary');
-      goog.require('Blockly.test.metrics');
-      goog.require('Blockly.test.mutator');
-      goog.require('Blockly.test.names');
-      goog.require('Blockly.test.procedures');
-      goog.require('Blockly.test.registry');
-      goog.require('Blockly.test.serialization');
-      goog.require('Blockly.test.shortcutRegistry');
-      goog.require('Blockly.test.theme');
-      goog.require('Blockly.test.toolbox');
-      goog.require('Blockly.test.tooltip');
-      goog.require('Blockly.test.trashcan');
-      goog.require('Blockly.test.utils');
-      goog.require('Blockly.test.variableMap');
-      goog.require('Blockly.test.variableModel');
-      goog.require('Blockly.test.variables');
-      goog.require('Blockly.test.widgetDiv');
-      goog.require('Blockly.test.workspaceComment');
-      goog.require('Blockly.test.workspaceSvg');
-      goog.require('Blockly.test.workspace');
-      goog.require('Blockly.test.xml');
-      goog.require('Blockly.test.zoomControls');
+	  // Test modules.
+	  'Blockly.test.astNode',
+	  'Blockly.test.blockChangeEvent',
+	  'Blockly.test.blockCreateEvent',
+	  'Blockly.test.blockJson',
+	  'Blockly.test.blocks',
+	  'Blockly.test.comments',
+	  'Blockly.test.commentDeserialization',
+	  'Blockly.test.connectionChecker',
+	  'Blockly.test.connectionDb',
+	  'Blockly.test.connection',
+	  'Blockly.test.contextMenuItem',
+	  'Blockly.test.cursor',
+	  'Blockly.test.dropdown',
+	  'Blockly.test.event',
+	  'Blockly.test.extensions',
+	  'Blockly.test.fieldAngle',
+	  'Blockly.test.fieldCheckbox',
+	  'Blockly.test.fieldColour',
+	  'Blockly.test.fieldDropdown',
+	  'Blockly.test.fieldImage',
+	  'Blockly.test.fieldLabelSerialization',
+	  'Blockly.test.fieldLabel',
+	  'Blockly.test.fieldMultiline',
+	  'Blockly.test.fieldNumber',
+	  'Blockly.test.fieldRegistry',
+	  'Blockly.test.fieldTest',
+	  'Blockly.test.fieldTextInput',
+	  'Blockly.test.fieldVariable',
+	  'Blockly.test.flyout',
+	  'Blockly.test.generator',
+	  'Blockly.test.gesture',
+	  'Blockly.test.input',
+	  'Blockly.test.insertionMarker',
+	  'Blockly.test.jsoDeserialization',
+	  'Blockly.test.jsoSerialization',
+	  'Blockly.test.json',
+	  'Blockly.test.keydown',
+	  'Blockly.test.lists',
+	  'Blockly.test.logicTernary',
+	  'Blockly.test.metrics',
+	  'Blockly.test.mutator',
+	  'Blockly.test.names',
+	  'Blockly.test.procedures',
+	  'Blockly.test.registry',
+	  'Blockly.test.serialization',
+	  'Blockly.test.shortcutRegistry',
+	  'Blockly.test.theme',
+	  'Blockly.test.toolbox',
+	  'Blockly.test.tooltip',
+	  'Blockly.test.trashcan',
+	  'Blockly.test.utils',
+	  'Blockly.test.variableMap',
+	  'Blockly.test.variableModel',
+	  'Blockly.test.variables',
+	  'Blockly.test.widgetDiv',
+	  'Blockly.test.workspaceComment',
+	  'Blockly.test.workspaceSvg',
+	  'Blockly.test.workspace',
+	  'Blockly.test.xml',
+	  'Blockly.test.zoomControls',
+	],
+	additionalScripts: [
+	  'msg/messages.js',
+	  'tests/playgrounds/screenshot.js',
+	  'node_modules/@blockly/dev-tools/dist/index.js',
+	],
+      }
     </script>
+    <script src="../bootstrap.js"></script>
 
     <div id="blocklyDiv"></div>
 
@@ -175,7 +178,10 @@
       <block type="test_field_block"></block>
     </xml>
 
-    <script>
+    <script type=module>
+      // Wait for Blockly to finish loading before running tests.
+      import '../bootstrap_done.mjs';
+
       let runner = mocha.run(function(failures) {
         var failureDiv = document.getElementById('failureCount');
         failureDiv.setAttribute('tests_failed', failures);
diff --git a/tests/multi_playground.html b/tests/multi_playground.html
index da48e2c85..21836cd6a 100644
--- a/tests/multi_playground.html
+++ b/tests/multi_playground.html
@@ -4,14 +4,14 @@
 <meta charset="utf-8">
 <title>Multi-toolbox Playground</title>
 
-<!-- This script loads uncompressed when on localhost and loads compressed when it is being hosted.
-     Please add any other dependencies to playgrounds/load_all.js.-->
-<script src="playgrounds/load_all.js"></script>
+<!-- This script loads uncompressed when on localhost and loads
+     compressed when it is being hosted or on Internet Explorer.  -->
+<script src="bootstrap.js"></script>
+<script type=module>
+// Wait for Blockly to finish loading.
+import './bootstrap_done.mjs';
 
-<script>
-'use strict';
-
-const IS_UNCOMPRESSED = !!document.getElementById('blockly-uncompressed-script');
+const IS_UNCOMPRESSED = !window.bootstrapInfo.compressed;  // See bootstrap.js
 
 var options = {
   comments: true,
@@ -74,6 +74,12 @@ function setBackgroundColour() {
     document.body.style.backgroundColor = '#60fcfc';  // Unfamiliar blue.
   }
 }
+
+// Call start().  Because this <script> has type=module, it is
+// automatically deferred, so it will not be run until after the
+// document has been parsed, but before firing DOMContentLoaded.
+start();
+
 </script>
 
 <style>
@@ -105,7 +111,7 @@ h1 {
 }
 </style>
 </head>
-<body onload="start()">
+<body>
   <h1>Blockly Multi Playground</h1>
 
   <form id="options">
diff --git a/tests/playground.html b/tests/playground.html
index 7a555392f..142eef3fb 100644
--- a/tests/playground.html
+++ b/tests/playground.html
@@ -4,15 +4,23 @@
 <meta charset="utf-8">
 <title>Blockly Playground</title>
 
-  <!-- This script loads uncompressed when on localhost and loads compressed when it is being hosted.
-       Please add any other dependencies to playgrounds/prepare.js.-->
-<script src="playgrounds/prepare.js"></script>
-<script src="playgrounds/screenshot.js"></script>
-<script src="../node_modules/@blockly/dev-tools/dist/index.js"></script>
+<!-- This script loads uncompressed when on localhost and loads
+     compressed when it is being hosted or on Internet Explorer.  -->
+<script>
+  var BLOCKLY_BOOTSTRAP_OPTIONS = {
+    additionalScripts: [
+      'msg/messages.js',
+      'tests/playgrounds/screenshot.js',
+      'node_modules/@blockly/dev-tools/dist/index.js',
+    ],
+  }
+</script>
+<script src="bootstrap.js"></script>
 <script type=module>
-import Blockly from './playgrounds/blockly.mjs';
+// Wait for Blockly to finish loading.
+import './bootstrap_done.mjs';
 
-const IS_UNCOMPRESSED = Boolean(window.BlocklyLoader);  // See prepare.js
+const IS_UNCOMPRESSED = !window.bootstrapInfo.compressed;  // See bootstrap.js
 var workspace = null;
 
 function start() {
@@ -385,10 +393,6 @@ var spaghettiXml = [
     'next': { }
   });
 
-// Call start().  Because this <script> has type=module, it is
-// automatically deferred, so it will not be run until after the
-// document has been parsed, but before firing DOMContentLoaded.
-
 function addEventHandlers() {
   document.getElementById('save-json').addEventListener('click', saveJson);
   document.getElementById('save-xml').addEventListener('click', saveXml);
@@ -426,6 +430,9 @@ function addEventHandlers() {
       .addEventListener('click', function() { workspace.setVisible(false); });
 }
   
+// Call start().  Because this <script> has type=module, it is
+// automatically deferred, so it will not be run until after the
+// document has been parsed, but before firing DOMContentLoaded.
 start();
 
 </script>
diff --git a/tests/playgrounds/advanced_playground.html b/tests/playgrounds/advanced_playground.html
index 1510555b7..0103b6961 100644
--- a/tests/playgrounds/advanced_playground.html
+++ b/tests/playgrounds/advanced_playground.html
@@ -3,15 +3,23 @@
 <head>
 <meta charset="utf-8">
 <title>Advanced Blockly Playground</title>
-<script src="prepare.js"></script>
-<script src="./screenshot.js"></script>
-<script src="../themes/test_themes.js"></script>
-<script src="../../node_modules/@blockly/dev-tools/dist/index.js"></script>
-<script src="../../node_modules/@blockly/theme-modern/dist/index.js"></script>
+
+<script>
+  var BLOCKLY_BOOTSTRAP_OPTIONS = {
+    additionalScripts: [
+      'msg/messages.js',
+      'tests/playgrounds/screenshot.js',
+      'tests/themes/test_themes.js',
+      'node_modules/@blockly/dev-tools/dist/index.js',
+      'node_modules/@blockly/theme-modern/dist/index.js',
+    ],
+  }
+</script>
+<script src="../bootstrap.js"></script>
 
 <script type="module">
-import Blockly from './blockly.mjs';
-'use strict';
+// Wait for Blockly to finish loading.
+import './bootstrap_done.mjs';
 
 function start() {
   setBackgroundColour();
diff --git a/tests/playgrounds/load_all.js b/tests/playgrounds/load_all.js
deleted file mode 100644
index 5eaa65e03..000000000
--- a/tests/playgrounds/load_all.js
+++ /dev/null
@@ -1,53 +0,0 @@
-/**
- * @license
- * Copyright 2021 Google LLC
- * SPDX-License-Identifier: Apache-2.0
- */
-
-/**
- * @fileoverview Loads uncompressed Blockly when running locally. Loads
- * compressed otherwise.
- */
-'use strict';
-
-
-/**
- * Loads all the compressed or uncompressed dependencies necessary to run the
- * playground. This is necessary since the goog.module conversion. Please see
- * issue #5557 for more information.
- */
-(function() {
-const isIe = navigator.userAgent.indexOf('MSIE') !== -1 ||
-    navigator.appVersion.indexOf('Trident/') > -1;
-
-if ((location.hostname === 'localhost' || location.hostname === '127.0.0.1' ||
-     location.hostname === '[::1]') &&
-    !isIe) {
-  document.write(
-      `<script src="../blockly_uncompressed.js" id="blockly-uncompressed-script"></script>`);
-  document.write(`<script src="../msg/messages.js"></script>`);
-  document.write(`<script src="../tests/themes/test_themes.js"></script>`);
-  document.write(
-      `<script src="../node_modules/@blockly/block-test/dist/index.js"></script>`);
-  document.write(`<script>
-                // Custom requires for the playground.
-                goog.require('Blockly.libraryBlocks');
-                goog.require('Blockly.Dart.all');
-                goog.require('Blockly.JavaScript.all');
-                goog.require('Blockly.Lua.all');
-                goog.require('Blockly.PHP.all');
-                goog.require('Blockly.Python.all');
-                goog.require('Blockly.WorkspaceCommentSvg');
-            </script>`);
-} else {
-  document.write(
-      `<script src="../blockly_compressed.js" id="blockly-compressed-script"></script>`);
-  document.write(`<script src="../blocks_compressed.js"></script>`);
-  document.write(`<script src="../dart_compressed.js"></script>`);
-  document.write(`<script src="../javascript_compressed.js"></script>`);
-  document.write(`<script src="../lua_compressed.js"></script>`);
-  document.write(`<script src="../php_compressed.js"></script>`);
-  document.write(`<script src="../python_compressed.js"></script>`);
-  document.write(`<script src="../msg/messages.js"></script>`);
-}
-})();
diff --git a/tests/playgrounds/prepare.js b/tests/playgrounds/prepare.js
deleted file mode 100644
index 5f17a97f7..000000000
--- a/tests/playgrounds/prepare.js
+++ /dev/null
@@ -1,99 +0,0 @@
-/**
- * @license
- * Copyright 2021 Google LLC
- * SPDX-License-Identifier: Apache-2.0
- */
-
-/**
- * @fileoverview Load this file in a <script> tag to prepare for
- *     importing Blockly into a web page.
- *
- *     You must use a <script> tag to load this script first, then
- *     import blockly.mjs in a <script type=module> to obtain the
- *     loaded value.
- *
- *     See tests/playground.html for example usage.
- */
-'use strict';
-
-(function() {
-  // Decide whether we can load Blockly uncompiled, or must load the
-  // compiled version.  Please see issue #5557 for more information.
-  const isIe = navigator.userAgent.indexOf('MSIE') !== -1 ||
-      navigator.appVersion.indexOf('Trident/') > -1;
-  const localhosts = ['localhost', '127.0.0.1', '[::1]'];
-
-  if (localhosts.includes(location.hostname) && !isIe) {
-    // We can load Blockly in uncompiled mode.
-
-    // Disable loading of closure/goog/deps.js (which doesn't exist).
-    window.CLOSURE_NO_DEPS = true;
-    // Load the Closure Library's base.js (the only part of the
-    // libary we use, mainly for goog.require / goog.provide /
-    // goog.module).
-    document.write('<script src="../../closure/goog/base.js"></script>');
-    // Load dependency graph info from build/deps.js.  To update
-    // deps.js, run `npm run build:deps`.
-    document.write('<script src="../../build/deps.js"></script>');
-
-    // Msg loading kludge.  This should go away once #5409 and/or
-    // #1895 are fixed.
-
-    // Load messages into a temporary Blockly.Msg object, deleting it
-    // afterwards (after saving the messages!)
-    window.Blockly = {Msg: Object.create(null)};
-    document.write('<script src="../../msg/messages.js"></script>');
-    document.write(`
-        <script>
-          window.BlocklyMsg = window.Blockly.Msg;
-          delete window.Blockly;
-        </script>`);
-
-    document.write(`
-        <script>
-          window.BlocklyLoader = new Promise((resolve, reject) => {
-            goog.bootstrap(
-                [
-                  'Blockly',
-                  'Blockly.libraryBlocks',
-                  'Blockly.Dart.all',
-                  'Blockly.JavaScript.all',
-                  'Blockly.Lua.all',
-                  'Blockly.PHP.all',
-                  'Blockly.Python.all',
-                ], resolve);
-          }).then(() => {
-            // Copy Messages from temporary Blockly.Msg object to the real one:
-            Object.assign(goog.module.get('Blockly').Msg, window.BlocklyMsg);
-          }).then(() => {
-            return goog.module.get('Blockly');
-          });
-        </script>`);
-  } else {
-    // The below code is necessary for a few reasons:
-    // - We need an absolute path instead of relative path because the
-    //   advanced_playground the and regular playground are in different folders.
-    // - We need to get the root directory for blockly because it is
-    //   different for github.io, appspot and local.
-    const files = [
-      'blockly_compressed.js',
-      'msg/messages.js',
-      'blocks_compressed.js',
-      'dart_compressed.js',
-      'javascript_compressed.js',
-      'lua_compressed.js',
-      'php_compressed.js',
-      'python_compressed.js',
-    ];
-
-    // We need to load Blockly in compiled mode.
-    const hostName = window.location.host.replaceAll('.', '\\.');
-    const matches = new RegExp(hostName + '\\/(.*)tests').exec(window.location.href);
-    const root = matches && matches[1] ? matches[1] : '';
-
-    // Load blockly_compressed.js et al. using <script> tags.
-    for (let i = 0; i < files.length; i++) {
-      document.write('<script src="/' + root + files[i] + '"></script>');
-    }
-  }
-})();