When using pub serve transformer can hang when using an invalid url.

Example templateUrl: '/gallery/gallery.html', (Which actually is a valid pub url but the transformer can't handle it now will file a separate issue for that.)

Exception:
Build completed successfully
in ShutdownIsolate: Unhandled exception:
Uncaught Error: The null object does not have a getter 'package'.

NoSuchMethodError: method not found: 'package'
Receiver: null
Arguments: []
Stack Trace:
#0 Object._noSuchMethod (dart:core-patch/object_patch.dart:42)
#1 Object.noSuchMethod (dart:core-patch/object_patch.dart:45)
#2 serializeId (package:$pub/serialize.dart:24:47)
#3 ForeignTransform.getInput (package:$pub/serialize/transform.dart:121:13)
#4 _ForeignBaseTransform&GetInputTransform.hasInput (package:$pub/serialize/get_input_transform.dart:29:12)
#5 _TransformAssetReader.hasInput (package:angular2/src/transform/common/asset_reader.dart:27:42)
#6 XhrImpl.get.<get_async_body> (package:angular2/src/transform/common/xhr_impl.dart:19:40)
#7 Future.Future.microtask. (dart:async/future.dart:144)
#8 _microtaskLoop (dart:async/schedule_microtask.dart:43)
#9 _microtaskLoopEntry (dart:async/schedule_microtask.dart:52)
#10 _runPendingImmediateCallback (dart:isolate-patch/isolate_patch.dart:96)
#11 _RawReceivePortImpl._handleMessage (dart:isolate-patch/isolate_patch.dart:149)
#0 _rootHandleUncaughtError. (dart:async/zone.dart:895)
#1 _microtaskLoop (dart:async/schedule_microtask.dart:43)
#2 _microtaskLoopEntry (dart:async/schedule_microtask.dart:52)
#3 _runPendingImmediateCallback (dart:isolate-patch/isolate_patch.dart:96)
#4 _RawReceivePortImpl._handleMessage (dart:isolate-patch/isolate_patch.dart:149)