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_annotations_files/expected/index.bootstrap.dart

@@ -0,0 +1,18 @@
+import 'package:angular2/src/reflection/reflection.dart' show reflector;
+import 'bar.dart' as i0;
+import 'package:angular2/src/core/annotations/annotations.dart' as i1;
+import 'package:angular2/src/core/annotations/template.dart' as i2;
+import 'index.dart' as i3;
+
+main() {
+  reflector
+    ..registerType(i0.MyComponent, {
+      "factory": () => new i0.MyComponent(),
+      "parameters": const [const []],
+      "annotations": const [
+        const i1.Component(selector: '[soup]'),
+        const i2.Template(inline: 'Salad')
+      ]
+    });
+  i3.main();
+}