watch the registration queue and print if it has no progress

pkg/polymer/lib/src/loader.dart

 // 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.
 
 part of polymer;
 
 /// Annotation used to automatically register polymer elements.
 class CustomTag {
   final String tagName;
   const CustomTag(this.tagName);
@@ skipping 50 matching lines @@
         'Please check that the list of entry points in your pubspec.yaml '
         'is correct. If you are using pub-serve, you may need to restart it.';
   }
 
   Polymer.registerSync('auto-binding-dart', AutoBindingElement,
       extendsTag: 'template');
 
   for (var initializer in initializers) {
     initializer();
   }
+
+  _watchWaitingFor();
 }
 
 /// Configures [initPolymer] making it optimized for deployment to the internet.
 /// With this setup the initializer list is supplied instead of searched for
 /// at runtime. Additionally, after this method is called [initPolymer] omits
 /// the [Zone] that automatically invokes [Observable.dirtyCheck].
 void configureForDeployment(List<Function> initializers) {
   loader.initializers = initializers;
   loader.deployMode = true;
 }
@@ skipping 72 matching lines @@
     return;
   }
 
   // Disable the loggers that were not specified.
   loggers.where((logger) => logFlags[logger.name] != true)
       .forEach((logger) {logger.level = Level.OFF;});
 
   // Listen to the polymer logs and print them to the console.
   polymerLogger.onRecord.listen((rec) {print(rec);});
 }
+
+/// Watches the waitingFor queue and if it fails to make progress then prints
+/// a message to the console.
+void _watchWaitingFor() {
+  int lastWaiting = Polymer.waitingFor.length;
+  int lastAlert;
+  new Timer.periodic(new Duration(seconds: 1), (Timer timer) {
+    var waiting = Polymer.waitingFor;
+    // Done, cancel timer.
+    if (waiting.isEmpty) {
+      timer.cancel();
+      return;
+    }
+    // Made progress, don't alert.
+    if (waiting.length != lastWaiting) {
+      lastWaiting = waiting.length;
+      return;
+    }
+    // Only alert once per waiting state.
+    if (lastAlert == lastWaiting) return;
+    lastAlert = lastWaiting;
+
+    print('No elements registered in a while, but still waiting on '
+        '${waiting.length} elements to be registered. Check that you have a '
+        'class with an @CustomTag annotation for each of the following tags: '
+        '${waiting.map((e) => "'${e.attributes['name']}'").join(', ')}');
+  });
+}