refactor(render): cleanup access to native dom elements

BREAKING CHANGES:
- rename `ElementRef.domElement` to `ElementRef.nativeElement`
- add `Renderer.getNativeElementSync` to make the app side
  less dependent on the dom renderer.
- don’t use `ElementRef.nativeElement` in directives but
  use the methods on `Renderer` directly.
- Removed `ElementRef.setAttribute`. Use `Renderer.setElementAttribute` instead.

Closes #2712
Last part of #2476
Closes #2476

modules/angular2/src/render/api.ts

@@ -370,6 +370,12 @@ export class Renderer {
    */
   dehydrateView(viewRef: RenderViewRef) {}
 
+  /**
+   * Returns the native element at the given location.
+   * Attention: In a WebWorker scenario, this should always return null!
+   */
+  getNativeElementSync(location: RenderElementRef): any { return null; }
+
   /**
    * Sets a property on an element.
    */