Adds support for scaling sprite physics groups

examples/game/test_physics.dart

@@ -51,6 +51,8 @@ class TestBed extends NodeWithSize {
 
   TestBed() : super(new Size(1024.0, 1024.0)) {
     _physicsNode = new PhysicsWorld(new Offset(0.0, 100.0));
+    PhysicsGroup group = new PhysicsGroup();
+    _physicsNode.addChild(group);
 
     _obstacle = new Sprite(_spriteSheet["ship.png"]);
     _obstacle.position = new Point(512.0, 800.0);
@@ -62,7 +64,7 @@ class TestBed extends NodeWithSize {
       friction: 0.5,
       tag: "obstacle"
     );
-    _physicsNode.addChild(_obstacle);
+    group.addChild(_obstacle);
     _physicsNode.addContactCallback(myCallback, "obstacle", "ship", PhysicsContactType.begin);
 
     // Animate obstacle

packages/flutter_sprites/lib/src/node.dart

@@ -277,15 +277,27 @@ class Node {
   void set scale(double scale) {
     assert(scale != null);
 
-    if (_physicsBody != null && parent is PhysicsWorld) {
-      PhysicsWorld physicsNode = parent;
-      physicsNode._updateScale(this.physicsBody, scale);
+    if (_physicsBody != null && (parent is PhysicsWorld || parent is PhysicsGroup)) {
+      _updatePhysicsScale(physicsBody, scale, parent);
     }
 
     _scaleX = _scaleY = scale;
     invalidateTransformMatrix();
   }
 
+  void _updatePhysicsScale(PhysicsBody body, double scale, Node physicsParent) {
+    if (physicsParent == null) return;
+
+    if (physicsParent is PhysicsWorld) {
+      PhysicsWorld world = physicsParent;
+      world._updateScale(body, scale);
+    } else if (physicsParent is PhysicsGroup) {
+      _updatePhysicsScale(body, scale * physicsParent.scale, physicsParent.parent);
+    } else {
+      assert(false);
+    }
+  }
+
   /// The horizontal scale of this node relative its parent.
   ///
   ///     myNode.scaleX = 5.0;