chore(build): Remove .es6 files which shadow .ts files.

This removes .es6 files which are pure duplicates of a
.ts file in the same folder.
Next we need to remove .js files as well, and remove karma preprocessors for dart.

test-main.js

@@ -17,7 +17,7 @@ System.paths = {
   'benchpress/*': 'dist/js/dev/es5/benchpress/*.js',
   'angular2/*': 'dist/js/dev/es5/angular2/*.js',
   'rtts_assert/*': 'dist/js/dev/es5/rtts_assert/*.js',
-  'rx/*': 'node_modules/rx/*.js'
+  'rx': 'node_modules/rx/dist/rx.js'
 };
 
 // Import all the specs, execute their `main()` method and kick off Karma (Jasmine).