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

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.

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/**'] : [];