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/angular2/src/transform/template_compiler/directive_metadata_reader.dart

@@ -19,8 +19,6 @@ num _getDirectiveType(String annotationName) {
       return DirectiveMetadata.DECORATOR_TYPE;
     case 'Component':
       return DirectiveMetadata.COMPONENT_TYPE;
-    case 'Viewport':
-      return DirectiveMetadata.VIEWPORT_TYPE;
     default:
       return -1;
   }