anchor.diff:    }
   }
   
  +abstract class _FakeMap {
  +  get isEmpty => throw "Not implemented";
  +  get isNotEmpty => throw "Not implemented";
  +  get keys => throw "Not implemented";
  +  get values => throw "Not implemented";
  +  get length => throw "Not implemented";
  +  operator[]=(_a, _b) => throw "Not implemented";
  +  addAll(_) => throw "Not implemented";
  +  clear() => throw "Not implemented";
  +  containsValue(_) => throw "Not implemented";
  +  forEach(_) => throw "Not implemented";
  +  putIfAbsent(_a, _b) => throw "Not implemented";
  +  remove(_) => throw "Not implemented";
  +}
  +
   /// 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 {
  +abstract class GlobalView extends Observable with _FakeMap implements Map {
     /// A real global view backed by a map of registered globals.
     factory GlobalView(EntityWrapper entity,
         Map<String, GlobalRegistration> globals) =>
  
positive.code: }
  
  /// A [GlobalView] for an [Entity] that does not have globals.
  class _EmptyGlobalView implements GlobalView {
  
    static const _singleton = const _EmptyGlobalView._useFactoryInstead();
  
  
negative.code:   operator[](String key);
  }
  
  class _GlobalViewImpl extends ChangeNotifier with _FakeMap implements GlobalView {
  
    EntityWrapper _entity;
    Map<String, GlobalRegistration> _globals;
  