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/test/services/ruler_spec.ts

@@ -46,7 +46,7 @@ export function main() {
        inject([AsyncTestCompleter], (async) => {
          var ruler = new Ruler(DOM);
          var elRef = <any>new SpyElementRef();
-         elRef.domElement = DOM.createElement('div');
+         elRef.nativeElement = DOM.createElement('div');
          ruler.measure(elRef).then((rect) => {
            // here we are using an element created in a doc fragment so all the measures will come
            // back as 0
@@ -61,7 +61,7 @@ export function main() {
 @proxy
 @IMPLEMENTS(ElementRef)
 class SpyElementRef extends SpyObject {
-  domElement;
+  nativeElement;
   constructor() { super(ElementRef); }
   noSuchMethod(m) { return super.noSuchMethod(m) }
 }