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/playground/src/svg/index.ts

@@ -1,4 +1,4 @@
-import {bootstrap} from 'angular2/bootstrap';
+import {bootstrap} from 'angular2/platform/browser';
 import {Component} from 'angular2/core';
 
 @Component({selector: '[svg-group]', template: `<svg:text x="20" y="20">Hello</svg:text>`})