anchor.diff:  import 'behavior.dart';
   import 'resolver.dart' show NativeClassFinder;
   
  -/// This could be an abstract class but we use it as a stub for the
  -/// dart_backend.
  -class NativeEnqueuer {
  -  /// Called when a [type] has been instantiated natively.
  -  void onInstantiatedType(InterfaceType type) {}
  -
  -  /// Initial entry point to native enqueuer.
  -  WorldImpact processNativeClasses(Iterable<Uri> libraries) =>
  -      const WorldImpact();
  -
  -  /// Registers the [nativeBehavior]. Adds the liveness of its instantiated
  -  /// types to the world.
  -  void registerNativeBehavior(
  -      WorldImpactBuilder impactBuilder, NativeBehavior nativeBehavior, cause) {}
  -
  -  /// Returns whether native classes are being used.
  -  bool get hasInstantiatedNativeClasses => false;
  -
  -  /// Emits a summary information using the [log] function.
  -  void logSummary(void log(String message)) {}
  -}
  -
  -abstract class NativeEnqueuerBase implements NativeEnqueuer {
  +abstract class NativeEnqueuer {
     final Set<ClassEntity> _registeredClasses = {};
     final Set<ClassEntity> _unusedClasses = {};
   
  -  @override
  +  /// Returns whether native classes are being used.
     bool get hasInstantiatedNativeClasses => !_registeredClasses.isEmpty;
   
     /// Log message reported if all native types are used.
  
positive.code:   }
  }
  
  class NativeResolutionEnqueuer extends NativeEnqueuerBase {
    final NativeClassFinder _nativeClassFinder;
  
    /// The set of all native classes.  Each native class is in [nativeClasses]
  
negative.code:   final CommonElements _commonElements;
  
    /// Subclasses of [NativeEnqueuerBase] are constructed by the backend.
    NativeEnqueuer(this._options, this._elementEnvironment, this._commonElements,
        this._dartTypes);
  
    bool get enableLiveTypeAnalysis => _options.enableNativeLiveTypeAnalysis;
  
    /// Called when a [type] has been instantiated natively.
    void onInstantiatedType(InterfaceType type) {
      if (_unusedClasses.remove(type.element)) {
        _registeredClasses.add(type.element);
      }
    }
  
    /// Initial entry point to native enqueuer.
    WorldImpact processNativeClasses(Iterable<Uri> libraries);
  
    /// Register [classes] as natively instantiated in [impactBuilder].
    void _registerTypeUses(
  