anchor.diff:      );
       _physicsNode.addChild(_obstacle);
       _physicsNode.addContactCallback(myCallback, "obstacle", "ship", PhysicsContactType.begin);
  +
  +    // Animate obstacle
  +    ActionSequence seq = new ActionSequence([
  +      new ActionTween((a) => _obstacle.position = a, new Point(256.0, 800.0), new Point(768.0, 800.0), 1.0, easeInOut),
  +      new ActionTween((a) => _obstacle.position = a, new Point(768.0, 800.0), new Point(256.0, 800.0), 1.0, easeInOut)
  +    ]);
  +    _obstacle.actions.run(new ActionRepeatForever(seq));
   
       addChild(_physicsNode);
   
  
positive.code: import 'dart:ui';
  
  import 'package:flutter/material.dart';
  import 'package:flutter/rendering.dart';
  import 'package:flutter/services.dart';
  
negative.code:     _physicsNode = new PhysicsNode(new Offset(0.0, 100.0));
  
      _obstacle = new Sprite(_spriteSheet["ship.png"]);
      _obstacle.position = new Point(512.0, 800.0);
      _obstacle.size = new Size(64.0, 64.0);
      _obstacle.physicsBody = new PhysicsBody(
        new PhysicsShapeCircle(Point.origin, 32.0),
  