anchor.diff:    /// A global view that doesn't have any globals.
     factory GlobalView.empty() => new _EmptyGlobalView();
   
  +  bool containsKey(String key);
     operator[](String key);
   }
   
  -class _GlobalViewImpl extends ChangeNotifier implements GlobalView {
  +class _GlobalViewImpl extends ChangeNotifier implements GlobalView, Map {
   
     EntityWrapper _entity;
     Map<String, GlobalRegistration> _globals;
  
positive.code:     return c.stream;
    }
  
    bool deliverChanges() {
    }
  
    bool get hasObservers => false;
  
    void notifyChange(ChangeRecord record) {
    }
  
    notifyPropertyChange(Symbol field, Object oldValue, Object newValue) {
    }
  }
  
negative.code: 
  /// A view of globals as they relate to a specific [Entity]. Implements
  /// observability based on dependencies of the globals involved.
  abstract class GlobalView extends Observable implements Map {
    /// A real global view backed by a map of registered globals.
    factory GlobalView(EntityWrapper entity,
        Map<String, GlobalRegistration> globals) =>
  