GlobalKeys should preserve state across tree mutations

This patch causes widgets with global keys to drag their state (including their
children) with them as they travel through the element tree.

packages/unit/test/widget/reparent_state_test.dart

+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:sky/widgets.dart';
+import 'package:test/test.dart';
+
+import 'widget_tester.dart';
+
+class StateMarker extends StatefulComponent {
+  StateMarker({ Key key, this.child }) : super(key: key);
+
+  final Widget child;
+
+  StateMarkerState createState() => new StateMarkerState();
+}
+
+class StateMarkerState extends State<StateMarker> {
+  String marker;
+
+  Widget build(BuildContext context) {
+    if (config.child != null)
+      return config.child;
+    return new Container();
+  }
+}
+
+void main() {
+  test('can reparent state', () {
+    testWidgets((WidgetTester tester) {
+      GlobalKey left = new GlobalKey();
+      GlobalKey right = new GlobalKey();
+
+      StateMarker grandchild = new StateMarker();
+      tester.pumpWidget(
+        new Stack([
+          new Container(
+            child: new StateMarker(key: left)
+          ),
+          new Container(
+            child: new StateMarker(
+              key: right,
+              child: grandchild
+            )
+          ),
+        ])
+      );
+
+      (left.currentState as StateMarkerState).marker = "left";
+      (right.currentState as StateMarkerState).marker = "right";
+
+      StateMarkerState grandchildState = tester.findStateByConfig(grandchild);
+      expect(grandchildState, isNotNull);
+      grandchildState.marker = "grandchild";
+
+      StateMarker newGrandchild = new StateMarker();
+      tester.pumpWidget(
+        new Stack([
+          new Container(
+            child: new StateMarker(
+              key: right,
+              child: newGrandchild
+            )
+          ),
+          new Container(
+            child: new StateMarker(key: left)
+          ),
+        ])
+      );
+
+      expect((left.currentState as StateMarkerState).marker, equals("left"));
+      expect((right.currentState as StateMarkerState).marker, equals("right"));
+
+      StateMarkerState newGrandchildState = tester.findStateByConfig(newGrandchild);
+      expect(newGrandchildState, isNotNull);
+      expect(newGrandchildState, equals(grandchildState));
+      expect(newGrandchildState.marker, equals("grandchild"));
+
+      tester.pumpWidget(
+        new Center(
+          child: new Container(
+            child: new StateMarker(
+              key: left,
+              child: new Container()
+            )
+          )
+        )
+      );
+
+      expect((left.currentState as StateMarkerState).marker, equals("left"));
+      expect(right.currentState, isNull);
+    });
+  });
+
+  test('can with scrollable list', () {
+    testWidgets((WidgetTester tester) {
+      GlobalKey key = new GlobalKey();
+
+      tester.pumpWidget(new StateMarker(key: key));
+
+      (key.currentState as StateMarkerState).marker = "marked";
+
+      tester.pumpWidget(new ScrollableList<int>(
+        items: [0],
+        itemExtent: 100.0,
+        itemBuilder: (BuildContext context, int item) {
+          return new Container(
+            key: new Key('container'),
+            height: 100.0,
+            child: new StateMarker(key: key)
+          );
+        }
+      ));
+
+      expect((key.currentState as StateMarkerState).marker, equals("marked"));
+
+      tester.pumpWidget(new StateMarker(key: key));
+
+      expect((key.currentState as StateMarkerState).marker, equals("marked"));
+    });
+  });
+}