Instead of applying a transform for every RenderObject, pass down an Offset for where to paint.

sky/engine/core/painting/Offset.dart

Index: sky/engine/core/painting/Offset.dart
diff --git a/sky/engine/core/painting/Offset.dart b/sky/engine/core/painting/Offset.dart
index 161bd754c5ca2637d09219f4bb6eea5fb4ab1cf9..13e458882e9090e44dc9bf333112a41da45cc599 100644
--- a/sky/engine/core/painting/Offset.dart
+++ b/sky/engine/core/painting/Offset.dart
@@ -20,7 +20,13 @@ class Offset extends OffsetBase {
 
   Offset operator -() => new Offset(-dx, -dy);
   Offset operator -(Offset other) => new Offset(dx - other.dx, dy - other.dy);
-  Offset operator +(Offset other) => new Offset(dx + other.dx, dy + other.dy);
+  dynamic operator +(dynamic other) {
+    if (other is Offset)
+      return new Offset(dx + other.dx, dy + other.dy);
+    if (other is Size)
+      return new Rect.fromLTWH(dx, dy, other.width, other.height);

[abarth-chromium, 2015/06/27 00:39:37]

Wow, an offset + a size is a rect?  Crazy.

+    throw new ArgumentError(other);
+  }
 
   // does the equivalent of "return new Point(0,0) + this"
   Point toPoint() => new Point(this.dx, this.dy);