feat(dart/transform): Use angular2/platform/browser as bootstrap lib

Update the Angular 2 transformer to recognize
`package:angular2/platform/browser.dart` as the library which exports
the `bootstrap` function.

Update playground, examples, benchmarks, & tests to import bootstrap from
platform/browser.

Closes #7647

modules/angular2/examples/core/ts/prod_mode/prod_mode_example.ts

@@ -1,6 +1,6 @@
 // #docregion enableProdMode
 import {enableProdMode} from 'angular2/core';
-import {bootstrap} from 'angular2/bootstrap';
+import {bootstrap} from 'angular2/platform/browser';
 import {MyComponent} from './my_component';
 
 enableProdMode();