anchor.diff:  // found in the LICENSE file.
   
   import 'dart:async';
  -import 'dart:ui' as ui;
  +import 'dart:ui' show Point, Offset;
   
   import 'arena.dart';
   import 'constants.dart';
  
positive.code: 
    _TapTracker({ PointerInputEvent event, this.entry })
      : pointer = event.pointer,
        _initialPosition = event.position,
        _isTrackingPointer = false {
      assert(event.type == 'pointerdown');
    }
  
    final int pointer;
    GestureArenaEntry entry;
    ui.Point _initialPosition;
    bool _isTrackingPointer;
  
    void startTrackingPointer(PointerRouter router, PointerRoute route) {
      if (!_isTrackingPointer) {
  
negative.code: import 'pointer_router.dart';
  import 'recognizer.dart';
  
  typedef void GestureTapDownCallback(Point globalPosition);
  typedef void GestureTapUpCallback(Point globalPosition);
  typedef void GestureTapCallback();
  typedef void GestureTapCancelCallback();
  
  