anchor.diff:      _physicsNode = new PhysicsNode(new Offset(0.0, 100.0));
   
       _obstacle = new Sprite(_spriteSheet["ship.png"]);
  -    _obstacle.position = new Point(532.0, 800.0);
  +    _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),
  
positive.code:     );
      _physicsNode.addChild(_obstacle);
      _physicsNode.addContactCallback(myCallback, "obstacle", "ship", PhysicsContactType.begin);
  
      addChild(_physicsNode);
  
  
negative.code: import 'dart:ui';
  
  import 'package:flutter/animation.dart';
  import 'package:flutter/material.dart';
  import 'package:flutter/rendering.dart';
  import 'package:flutter/services.dart';
  