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/render/dom/dom_renderer_integration_spec.ts

@@ -14,7 +14,7 @@ import {
 } from 'angular2/testing_internal';
 
 // import {MapWrapper} from 'angular2/src/facade/collection';
-// import {DOM} from 'angular2/src/core/dom/dom_adapter';
+// import {DOM} from 'angular2/src/platform/dom/dom_adapter';
 
 // import {DomTestbed, TestRootView, elRef} from './dom_testbed';
 

modules/angular2/test/core/render/dom/events/event_manager_spec.ts

@@ -13,7 +13,7 @@ import {EventManager, EventManagerPlugin} from 'angular2/core';
 import {DomEventsPlugin} from 'angular2/src/platform/dom/events/dom_events';
 import {NgZone} from 'angular2/src/core/zone/ng_zone';
 import {ListWrapper, Map, MapWrapper} from 'angular2/src/facade/collection';
-import {DOM} from 'angular2/src/core/dom/dom_adapter';
+import {DOM} from 'angular2/src/platform/dom/dom_adapter';
 
 export function main() {
   var domEventPlugin;

modules/angular2/test/core/render/dom/shared_styles_host_spec.ts

@@ -16,7 +16,7 @@ import {
   proxy
 } from 'angular2/testing_internal';
 
-import {DOM} from 'angular2/src/core/dom/dom_adapter';
+import {DOM} from 'angular2/src/platform/dom/dom_adapter';
 import {DomSharedStylesHost} from 'angular2/src/platform/dom/shared_styles_host';
 
 export function main() {

modules/angular2/test/core/render/view_factory_spec.ts

@@ -25,7 +25,7 @@ import {
   RenderComponentTemplate
 } from 'angular2/src/core/render/api';
 import {SpyRenderEventDispatcher} from '../spies';
-import {DOM} from 'angular2/src/core/dom/dom_adapter';
+import {DOM} from 'angular2/src/platform/dom/dom_adapter';
 import {ViewEncapsulation} from 'angular2/src/core/metadata';
 
 function beginElement(name: string, attrNameAndValues: string[], eventTargetAndNames: string[],