chore(build): rename .es6 files to .js

Change es6 source files in npm distribution to use .js extensions.

Closes #2447

docs/dgeni-package/services/tsParser.js

@@ -9,7 +9,7 @@ module.exports = function tsParser(createCompilerHost, log) {
     // During migration from Traceur, there is a mix of `.ts`, `.es6` and `.js` (atScript)
     // files in the project and the TypeScript compiler only looks for `.ts` files when trying
     // to load imports.
-    extensions: ['.ts', '.js', '.es6'],
+    extensions: ['.ts', '.js'],
 
     // The options for the TS compiler
     options: {