chore(build): don’t include export var __esModule = true in every file

But do it during the build process for cjs.
Right now we only need this when we transpile from ts
directly to es5. This is only the case in our
cis build, as for our browser build we only transpile
from ts to es6 via ts and then use traceur to do
the rest.

modules/angular2/src/change_detection/pipes/lowercase_pipe.ts

@@ -1,11 +1,6 @@
 import {isString, StringWrapper} from 'angular2/src/facade/lang';
 import {Pipe} from './pipe';
 
-// HACK: workaround for Traceur behavior.
-// It expects all transpiled modules to contain this marker.
-// TODO: remove this when we no longer use traceur
-export var __esModule = true;
-
 /**
  * Implements lowercase transforms to text.
  *