diff --git a/tests/bootstrap.js b/tests/bootstrap.js
index e0cf0115e..fb185e1e2 100644
--- a/tests/bootstrap.js
+++ b/tests/bootstrap.js
@@ -125,11 +125,11 @@
         '  window.BlocklyMsg = window.Blockly.Msg;\n' +
         '  delete window.Blockly;\n' +
         '</script>\n');
-    const requires = options.requires.map(r => '\'' + r + '\'').join();
+    const allRequires = options.requires.map(quote).join();
     document.write(
         '<script>\n' +
         '  window.BlocklyLoader = new Promise((resolve, reject) => {\n' +
-        '    goog.bootstrap([' + requires + '], resolve);\n' +
+        '    goog.bootstrap([' + allRequires + '], resolve);\n' +
         '  }).then(() => {\n' +
         '    // Copy Messages from temp Blockly.Msg object to the real one:\n' +
         '    Object.assign(goog.module.get(\'Blockly\').Msg,\n' +
@@ -147,4 +147,47 @@
           '<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) + "'";
+  }
 })();