Move examples to //sky/sdk/example

sky/sdk/example/game/lib/game_demo_world.dart

 part of game;
 
 const double _steeringThreshold = 0.0;
 const double _steeringMax = 150.0;
 
 // Random generator
 Math.Random _rand = new Math.Random();
 
 const double _gameSizeWidth = 1024.0;
 const double _gameSizeHeight = 1024.0;
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   double dy = a.y - b.y;
   if (dx < 0.0) dx = -dx;
   if (dy < 0.0) dy = -dy;
   if (dx > dy) {
     return dx + dy/2.0;
   }
   else {
     return dy + dx/2.0;
   }
 }