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/examples/src/hello_world/index_common.ts

@@ -1,4 +1,4 @@
-import {ElementRef, Component, Directive, View, Injectable} from 'angular2/angular2';
+import {ElementRef, Component, Directive, View, Injectable, Renderer} from 'angular2/angular2';
 
 // A service available to the Injector, used by the HelloCmp component.
 @Injectable()
@@ -12,7 +12,7 @@ class GreetingService {
 class RedDec {
   // ElementRef is always injectable and it wraps the element on which the
   // directive was found by the compiler.
-  constructor(el: ElementRef) { el.domElement.style.color = 'red'; }
+  constructor(el: ElementRef, renderer: Renderer) { renderer.setElementStyle(el, 'color', 'red'); }
 }
 
 // Angular 2.0 supports 2 basic types of directives: