feat(dart/transform) Allow ctor stubs to be tree shaken

Change the method used to discover Directive annotated classes to ensure
that the Dart code can be tree shaken.

Closes #497

Closes $\x23736

modules/angular2/test/transform/simple_annotation_files/index.dart

@@ -1,7 +1,8 @@
 library web_foo;
 
+import 'package:angular2/src/core/application.dart';
 import 'bar.dart';
 
 void main() {
-  new MyComponent('Things');
+  bootstrap(MyComponent);
 }