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/platform/common_dom.ts

@@ -1,6 +1,7 @@
 /**
  * This is a set of classes and objects that can be used both in the browser and on the server.
  */
+export {DOM, setRootDomAdapter, DomAdapter} from 'angular2/src/platform/dom/dom_adapter';
 export {DomRenderer} from 'angular2/src/platform/dom/dom_renderer';
 export {DOCUMENT} from 'angular2/src/platform/dom/dom_tokens';
 export {SharedStylesHost, DomSharedStylesHost} from 'angular2/src/platform/dom/shared_styles_host';