fix(build): Correctly handle deep export paths in UMD wrapper (#5945)

* fix(build): Correctly handle deep export paths

A problem can occur when loading chunks in a browser: although
factory() will create the full exported path on $, and thus the
assignment

    root.<path> = factory(...) { ...; return $.<path> }

will normally be a do-nothing in every chunk except the first, if
the exported path (e.g. Blockly.blocks.all) is more than one level
deeper than any previously-existing path (e.g. Blockly), this will
fail because root.<path> is evaluated before calling factory(), and
so the left hand side will will evaluate to a undefined reference
(e.g. undefined.all) and TypeError will be thrown before the call
to factory is evaluated.

To fix this, call factory first and store the exports object in a
variable before assigning it to the exported path.

Fixes #5932.

scripts/gulpfiles/build_tasks.js

@@ -333,6 +333,12 @@ function chunkWrapper(chunk) {
   const browserDeps =
       chunk.dependencies.map(d => `root.${d.exports}`).join(', ');
   const factoryParams = chunk.dependencies.map(d => d.importAs).join(', ');
+
+  // Note that when loading in a browser the base of the exported path
+  // (e.g. Blockly.blocks.all - see issue #5932) might not exist
+  // before factory has been executed, so calling factory() and
+  // assigning the result are done in separate statements to ensure
+  // they are sequenced correctly.
   return `// Do not edit this file; automatically generated.
 
 /* eslint-disable */
@@ -342,7 +348,8 @@ function chunkWrapper(chunk) {
   } else if (typeof exports === 'object') { // Node.js
     module.exports = factory(${cjsDeps});
   } else { // Browser
-    root.${chunk.exports} = factory(${browserDeps});
+    var factoryExports = factory(${browserDeps});
+    root.${chunk.exports} = factoryExports;
   }
 }(this, function(${factoryParams}) {
 ${chunk.factoryPreamble || FACTORY_PREAMBLE}