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/src/transform/logging.dart

@@ -0,0 +1,24 @@
+library angular2.src.transform.logging;
+
+import 'package:barback/barback.dart';
+import 'package:code_transformers/messages/build_logger.dart';
+
+BuildLogger _logger;
+
+/// Prepares [logger] for use throughout the transformer.
+void init(Transform t) {
+  if (_logger == null) {
+    _logger = new BuildLogger(t);
+  } else {
+    _logger.fine('Attempted to initialize logger multiple times.',
+        asset: t.primaryInput.id);
+  }
+}
+
+/// The logger the transformer should use for messaging.
+BuildLogger get logger {
+  if (_logger == null) {
+    throw new StateError('Logger never initialized.');
+  }
+  return _logger;
+}