Implement RenderImage and Image for Sky

sky/sdk/lib/framework/rendering/node.dart

Index: sky/sdk/lib/framework/rendering/node.dart
diff --git a/sky/sdk/lib/framework/rendering/node.dart b/sky/sdk/lib/framework/rendering/node.dart
index 45ed33921cd079e73fd46d4e78433a78432923a4..cfeec1599ddcfe35ef32316a6f407fd5990a3e7d 100644
--- a/sky/sdk/lib/framework/rendering/node.dart
+++ b/sky/sdk/lib/framework/rendering/node.dart
@@ -229,6 +229,14 @@ abstract class RenderNodeWithChildMixin<ChildType extends RenderNode> {
       adoptChild(_child);
     markNeedsLayout();
   }
+  void attachChildren() {
+    if (_child != null)
+      _child.attach();
+  }
+  void detachChildren() {
+    if (_child != null)
+      _child.detach();
+  }
 }