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.

tools/build/snippets/runtime_paths.js

@@ -1,6 +1,6 @@
 System.paths = {
   '*': '/*.js',
-  'rx/dist/*': '*.js'
+  'rx': 'rx.js'
 };
 register(System);
 cjs(System);