Provide some debug-mode time hooks to visually debug painting and layout.

sky/engine/core/painting/Rect.dart

Index: sky/engine/core/painting/Rect.dart
diff --git a/sky/engine/core/painting/Rect.dart b/sky/engine/core/painting/Rect.dart
index 85fe07331e05b24ac4583377b91de6b21ba90b5b..8a82e69a5666da44193f868ccba392882c6e41de 100644
--- a/sky/engine/core/painting/Rect.dart
+++ b/sky/engine/core/painting/Rect.dart
@@ -30,6 +30,13 @@ class Rect {
   double get right => _value[2];
   double get bottom => _value[3];
 
+  Rect shift(Offset offset) {
+    return new Rect.fromLTRB(left + offset.dx, top + offset.dy, right + offset.dx, bottom + offset.dy);
+  }
+  Rect inflate(double delta) {
+    return new Rect.fromLTRB(left - delta, top - delta, right + delta, bottom + delta);
+  }
+
   double get width => right - left;
   double get height => bottom - top;