refactor(build)!: Provide all generator exports when loaded as script (#7169)

Previously, when loading a generator chunk (e.g.,
javascript_compressed.js) as a script (e.g., in a browser using
a <SCRIPT> tag), only a single named export from that chunk would
be made available (e.g, javascriptGenerator would be made availabe
as Blockly.JavaScript).

Until recently, that was fine because each generator chunk had only
a single named export, but now each one additionally has a
<Lang>Generator class and Order enum export.

To allow these new exports to be accessed by script users, the
chunk wrappers are modified to provide the whole export object
at a correspondingly-named global variable—e.g., when loaded as
a script javascript_compressed.js creates a global variable named
javascript, so the named exports can be accessed as
javascript.javascriptGenerator, javascript.JavascriptGenerator
and javascript.Order, as if the user had imported them via

    import * as javascript from 'blockly/javascript';

This PR includes a breaking change and a deprecation, both of
which are only applicable when loading generators as scripts
(e.g. via a <SCRIPT> tag):

BREAKING CHANGE: The generator chunks will, when loaded as scripts
(e.g. via a <SCRIPT> tag, now clobber any existing global variable
of the corresponding name:

- dart_compresed.js will set dart
- javascript_compresed.js will set javascript
- lua_compresed.js will set lua
- php_compresed.js will set php
- python_compresed.js will set python

DEPRECATION: Accessing the generator instances at their previous
locations (Blockly.Dart, Blockly.JavaScript, Blockly.Lua,
Blockly.PHP, and Blockly.Python) is deprecated and may cease
to work in a future version of Blockly.

scripts/gulpfiles/build_tasks.js

@@ -75,12 +75,15 @@ const NAMESPACE_PROPERTY = '__namespace__';
  *   chunk.
  * - .exports: an expression evaluating to the exports/Module object
  *   of module that is the chunk's entrypoint / top level module.
- * - .reexport: if running in a browser, save the chunk's exports
- *   object (or a single export of it; see reexportOnly, below) at
- *   this location in the global namespace.
- * - .reexportOnly: if reexporting and this property is set,
- *   save only the correspondingly-named export.  Otherwise
- *   save the whole export object.
+ * - .scriptExport: When the chunk is loaded as a script (e.g., via a
+ *   <SCRIPT> tag), the chunk's exports object will be made available
+ *   at the specified location (which must be a variable name or the
+ *   name of a property on an already-existing object) in the global
+ *   namespace.
+ * - .scriptNamedExports: A map of {location: namedExport} pairs; when
+ *   loaded as a script, the specified named exports will be saved at
+ *   the specified locations (which again must be global variables or
+ *   properties on already-existing objects).  Optional.
  *
  * The function getChunkOptions will, after running
  * closure-calculate-chunks, update each chunk to add the following
@@ -97,48 +100,48 @@ const chunks = [
     name: 'blockly',
     entry: path.join(TSC_OUTPUT_DIR, 'core', 'main.js'),
     exports: 'module$build$src$core$blockly',
-    reexport: 'Blockly',
+    scriptExport: 'Blockly',
   },
   {
     name: 'blocks',
     entry: path.join(TSC_OUTPUT_DIR, 'blocks', 'blocks.js'),
     exports: 'module$exports$Blockly$libraryBlocks',
-    reexport: 'Blockly.libraryBlocks',
+    scriptExport: 'Blockly.libraryBlocks',
   },
   {
     name: 'javascript',
     entry: path.join(TSC_OUTPUT_DIR, 'generators', 'javascript', 'all.js'),
     exports: 'module$build$src$generators$javascript$all',
-    reexport: 'Blockly.JavaScript',
-    reexportOnly: 'javascriptGenerator',
+    scriptExport: 'javascript',
+    scriptNamedExports: {'Blockly.Javascript': 'javascriptGenerator'},
   },
   {
     name: 'python',
     entry: path.join(TSC_OUTPUT_DIR, 'generators', 'python', 'all.js'),
     exports: 'module$build$src$generators$python$all',
-    reexport: 'Blockly.Python',
-    reexportOnly: 'pythonGenerator',
+    scriptExport: 'python',
+    scriptNamedExports: {'Blockly.Python': 'pythonGenerator'},
   },
   {
     name: 'php',
     entry: path.join(TSC_OUTPUT_DIR, 'generators', 'php', 'all.js'),
     exports: 'module$build$src$generators$php$all',
-    reexport: 'Blockly.PHP',
-    reexportOnly: 'phpGenerator',
+    scriptExport: 'php',
+    scriptNamedExports: {'Blockly.PHP': 'phpGenerator'},
   },
   {
     name: 'lua',
     entry: path.join(TSC_OUTPUT_DIR, 'generators', 'lua', 'all.js'),
     exports: 'module$build$src$generators$lua$all',
-    reexport: 'Blockly.Lua',
-    reexportOnly: 'luaGenerator',
+    scriptExport: 'lua',
+    scriptNameExports: {'Blockly.Lua': 'luaGenerator'},
   },
   {
     name: 'dart',
     entry: path.join(TSC_OUTPUT_DIR, 'generators', 'dart', 'all.js'),
     exports: 'module$build$src$generators$dart$all',
-    reexport: 'Blockly.Dart',
-    reexportOnly: 'dartGenerator',
+    scriptExport: 'dart',
+    scriptNameExports: {'Blockly.Dart': 'dartGenerator'},
   }
 ];
 
@@ -441,7 +444,7 @@ function chunkWrapper(chunk) {
   // JavaScript expressions for the amd, cjs and browser dependencies.
   let amdDepsExpr = '';
   let cjsDepsExpr = '';
-  let browserDepsExpr = '';
+  let scriptDepsExpr = '';
   // Arguments for the factory function.
   let factoryArgs = '';
   // Expression to get or create the namespace object.
@@ -452,25 +455,22 @@ function chunkWrapper(chunk) {
         JSON.stringify(`./${chunk.parent.name}${COMPILED_SUFFIX}.js`);
     amdDepsExpr = parentFilename;
     cjsDepsExpr = `require(${parentFilename})`;
-    browserDepsExpr = `root.${chunk.parent.reexport}`;
+    scriptDepsExpr = `root.${chunk.parent.scriptExport}`;
     factoryArgs = '__parent__';
     namespaceExpr = `${factoryArgs}.${NAMESPACE_PROPERTY}`;
   }
 
-  // Code to assign the result of the factory function to the desired
-  // export location when running in a browser.  When
-  // chunk.reexportOnly is set, this additionally does two other
-  // things:
-  // - It ensures that only the desired property of the exports object
-  //   is assigned to the specified reexport location.
-  // - It ensures that the namesspace object is accessible via the
-  //   selected sub-object, so that any dependent modules can obtain
-  //   it.
-  const browserExportStatements = chunk.reexportOnly ?
-      `root.${chunk.reexport} = factoryExports.${chunk.reexportOnly};\n` +
-      `    root.${chunk.reexport}.${NAMESPACE_PROPERTY} = ` +
-      `factoryExports.${NAMESPACE_PROPERTY};` :
-      `root.${chunk.reexport} = factoryExports;`;
+  // Code to save the chunk's exports object at chunk.scriptExport and
+  // additionally save individual named exports as directed by
+  // chunk.scriptNamedExports.
+  const scriptExportStatements = [
+    `root.${chunk.scriptExport} = factory(${scriptDepsExpr});`,
+  ];
+  for (var location in chunk.scriptNamedExports) {
+    const namedExport = chunk.scriptNamedExports[location];
+    scriptExportStatements.push( 
+      `root.${location} = root.${chunk.scriptExport}.${namedExport};`);
+  }
 
   // Note that when loading in a browser the base of the exported path
   // (e.g. Blockly.blocks.all - see issue #5932) might not exist
@@ -485,9 +485,8 @@ function chunkWrapper(chunk) {
     define([${amdDepsExpr}], factory);
   } else if (typeof exports === 'object') { // Node.js
     module.exports = factory(${cjsDepsExpr});
-  } else { // Browser
-    var factoryExports = factory(${browserDepsExpr});
-    ${browserExportStatements}
+  } else { // Script
+    ${scriptExportStatements.join('\n    ')}
   }
 }(this, function(${factoryArgs}) {
 var ${NAMESPACE_VARIABLE}=${namespaceExpr};