Parent data fixes

Add some missing functionality to ParentData subclasses.

Fixes #474

packages/flutter/lib/src/rendering/custom_layout.dart

@@ -7,6 +7,14 @@ import 'object.dart';
 
 class MultiChildLayoutParentData extends ContainerBoxParentDataMixin<RenderBox> {
   Object id;
+
+  void merge(MultiChildLayoutParentData other) {
+    if (other.id != null)
+      id = other.id;
+    super.merge(other);
+  }
+
+  String toString() => '${super.toString()}; id=$id';
 }
 
 abstract class MultiChildLayoutDelegate {

packages/flutter/lib/src/rendering/stack.dart

@@ -192,7 +192,7 @@ class StackParentData extends ContainerBoxParentDataMixin<RenderBox> {
   /// children in the stack.
   bool get isPositioned => top != null || right != null || bottom != null || left != null || width != null || height != null;
 
-  String toString() => '${super.toString()}; top=$top; right=$right; bottom=$bottom, left=$left';
+  String toString() => '${super.toString()}; top=$top; right=$right; bottom=$bottom; left=$left; width=$width; height=$height';
 }
 
 abstract class RenderStackBase extends RenderBox