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/test/core/debug/debug_element_spec.ts

@@ -14,7 +14,7 @@ import {
   TestComponentBuilder
 } from 'angular2/testing_internal';
 
-import {DOM} from 'angular2/src/core/dom/dom_adapter';
+import {DOM} from 'angular2/src/platform/dom/dom_adapter';
 
 import {PromiseWrapper, EventEmitter, ObservableWrapper} from 'angular2/src/facade/async';