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/two_deps_files/expected/index.bootstrap.dart

@@ -10,9 +10,7 @@ main() {
       "factory":
           (i1.MyContext c, String inValue) => new i0.MyComponent(c, inValue),
       "parameters": const [const [i1.MyContext, String]],
-      "annotations": const [
-        const i2.Component(selector: i1.preDefinedSelector)
-      ]
+      "annotations": const [const i2.Component(selector: i1.preDefinedSelector)]
     });
   i3.main();
 }

modules/angular2/test/transform/two_deps_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);
 }