cleanup: move DomAdapter from angular2/core into angular2/platform/common_dom

BREAKING CHANGE

Before

import {DomAdapter} from 'angular2/core';

After

import {DomAdapter} from 'angular2/platform/common_dom';

Closes #5370

modules/angular2/angular2.ts

@@ -3,5 +3,6 @@ export * from './core';
 export * from './profile';
 export * from './lifecycle_hooks';
 export * from './platform/browser';
+export * from './src/platform/dom/dom_adapter';
 export * from './upgrade';
 export {UrlResolver, AppRootUrl} from './compiler';