Kill the "shadow-functions.js" internal plugin in favor of an explicit helper (#5677)

* Handle arrow function processing via shared API rather than default plugin.

* Fix a few small PR comments.

* Preserve existing spec arrow 'this' rewrites, and support spec in subclass constructors.

packages/babel-plugin-transform-es2015-modules-commonjs/src/index.js

@@ -139,8 +139,7 @@ export default function () {
 
         if (
           state.opts.allowTopLevelThis !== true &&
-          !path.findParent((path) => !path.is("shadow") &&
-          THIS_BREAK_KEYS.indexOf(path.type) >= 0)
+          !path.findParent((path) => THIS_BREAK_KEYS.indexOf(path.type) >= 0)
         ) {
           path.replaceWith(t.identifier("undefined"));
         }