Poor recovery when annotations are used where not allowed

[bwilkerson]

As an example, enter the following into a Dart file:

class EventTarget {
  EventTarget() : _eventsController = new DispatcherController<@nonnull Event>();
}

[bwilkerson]

Here is a more complete reproduction case:

const annotation = null;
class A<E> {}
class C {
  m() => new A<@annotation C>();
}

[srawlins]

I'm not sure what "poor recovery" meant at the time this bug was filed. Today only one error is issued:

$ dart 22314.dart 
22314.dart:4:16: Error: Unexpected token '@'.
  m() => new A<@annotation C>();
               ^

It could be better messaging ("No annotations here, mate").