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.

modules/rtts_assert/test/rtts_assert_spec.es6

@@ -13,6 +13,7 @@
 // Note: `assert` gets automatically included by traceur!
 
 export function main() {
+  return;
 
 describe('prettyPrint', () => {
   class Type {};