refactor(compiler): remove Viewport directives, use Decorator instead

BREAKING_CHANGE:
- The special type of `Viewport` directives is removed
  in favor of a more general `Decorator` directive
- `ViewContainerRef` now no more has a default `ProtoViewRef`
  but requires an explicit one when creating views.

Closes #1536

modules/benchmarks/src/tree/tree_benchmark.js

@@ -1,4 +1,4 @@
-import {bootstrap, Component, Viewport, View, ViewContainerRef, Compiler, Decorator} from 'angular2/angular2';
+import {bootstrap, Component, View, ViewContainerRef, Compiler, Decorator} from 'angular2/angular2';
 
 import {LifeCycle} from 'angular2/src/core/life_cycle/life_cycle';
 import {reflector} from 'angular2/src/reflection/reflection';