[BUGFIX release] Ensure loader.js is transpiled via targets.

Prior to this change, the `packages/loader/lib/index.js` file was not
transpiled (at all). However, until 4a5b372 landed the file was still
authored (manually) in IE11 compliant ES5 JavaScript. Once 4a5b372
landed however (first released in 3.14.0-beta.1) it is impossible to use
the built assets in a browser that does not support `let` variable
declaration (e.g. Safari
9).

This ensures that `packages/loader/lib/index.js` passes through the same
transpilation process as the rest of the application, and (confirmed
manually) fixes the issue reported.

Ironically, our CI did not catch this issue (even though we are properly
testing in IE11) because IE11 actually supports `let` usage (with minor
caveats).

Also, since `loader.js` is the "one thing" that does not need modules
transpilation, this also adds a new argument to `transpileTree`. Most of
the time that option is `undefined` (and therefore not used at all), but
for header files we need to avoid the module wrapping.

Author: Robert Jackson

lib/index.js

@@ -128,7 +128,7 @@ module.exports = {
     }
   },
 
-  transpileTree(tree, isProduction) {
+  transpileTree(tree, isProduction, shouldCompileModules) {
     let emberCliBabel = this.addons.find(a => a.name === 'ember-cli-babel');
 
     let parentOptions = this.parent && this.parent.options;
@@ -158,6 +158,13 @@ module.exports = {
       }),
     };
 
+    if (shouldCompileModules !== undefined) {
+      // ember-cli-babel internally uses **any** value that was provided IIF
+      // the option is set so this option must only be set when we have a
+      // useful value for it
+      options['ember-cli-babel'].compileModules = shouldCompileModules;
+    }
+
     if (isProduction) {
       options.babel.plugins.push(buildStripClassCallcheckPlugin());
     }
@@ -173,7 +180,11 @@ module.exports = {
       isProduction
     );
 
-    let headerFiles = new Funnel(tree, { srcDir: 'header' });
+    let headerFiles = this.transpileTree(
+      new Funnel(tree, { srcDir: 'header' }),
+      isProduction,
+      false
+    );
 
     let exclude = isProduction ? ['ember-testing/**'] : [];