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/common.dart

@@ -0,0 +1,14 @@
+// Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+library angular2.test.transform;
+
+import 'dart:async';
+
+/// Mocked out version of [bootstrap], defined in application.dart. Importing
+/// the actual file in tests causes issues with resolution due to its
+/// transitive dependencies.
+Future bootstrap(Type appComponentType, [List<Binding> bindings = null,
+    Function givenBootstrapErrorReporter = null]) {
+  return null;
+}