Introduce some basic utility classes.

One might argue, the most basic utility classes.

examples/stocks/lib/stock_menu.dart

@@ -9,6 +9,7 @@ enum _MenuItems { autorefresh, autorefreshCheckbox, refresh, speedUp, speedDown
 const double _kMenuMargin = 16.0; // 24.0 on tablet
 
 Future showStockMenu({BuildContext context, bool autorefresh, ValueChanged<bool> onAutorefreshChanged }) async {
+  StateSetter autorefreshStateSetter;
   switch (await showMenu(
     context: context,
     position: new ModalPosition(
@@ -20,12 +21,18 @@ Future showStockMenu({BuildContext context, bool autorefresh, ValueChanged<bool>
         value: _MenuItems.autorefresh,
         child: new Row(<Widget>[
             new Flexible(child: new Text('Autorefresh')),
-            new Checkbox(
-              value: autorefresh,
-              onChanged: (bool value) {
-                // TODO(ianh): https://github.com/flutter/flutter/issues/187
-                autorefresh = value;
-                Navigator.pop(context, _MenuItems.autorefreshCheckbox);
+            new StatefulBuilder(
+              builder: (BuildContext context, StateSetter setState) {
+                autorefreshStateSetter = setState;
+                return new Checkbox(
+                  value: autorefresh,
+                  onChanged: (bool value) {
+                    setState(() {
+                      autorefresh = value;
+                    });
+                    Navigator.pop(context, _MenuItems.autorefreshCheckbox);
+                  }
+                );
               }
             )
           ]
@@ -46,8 +53,9 @@ Future showStockMenu({BuildContext context, bool autorefresh, ValueChanged<bool>
     ]
   )) {
     case _MenuItems.autorefresh:
-      // TODO(ianh): https://github.com/flutter/flutter/issues/187
-      autorefresh = !autorefresh;
+      autorefreshStateSetter(() {
+        autorefresh = !autorefresh;
+      });
       continue autorefreshNotify;
     autorefreshNotify:
     case _MenuItems.autorefreshCheckbox:

packages/flutter/lib/src/widgets/basic.dart

@@ -1367,3 +1367,19 @@ class KeyedSubtree extends StatelessComponent {
 
   Widget build(BuildContext context) => child;
 }
+
+class Builder extends StatelessComponent {
+  Builder({ Key key, this.builder }) : super(key: key);
+  final WidgetBuilder builder;
+  Widget build(BuildContext context) => builder(context);
+}
+
+typedef Widget StatefulWidgetBuilder(BuildContext context, StateSetter setState);
+class StatefulBuilder extends StatefulComponent {
+  StatefulBuilder({ Key key, this.builder }) : super(key: key);
+  final StatefulWidgetBuilder builder;
+  _StatefulBuilderState createState() => new _StatefulBuilderState();
+}
+class _StatefulBuilderState extends State<StatefulBuilder> {
+  Widget build(BuildContext context) => config.builder(context, setState);
+}
\ No newline at end of file

packages/unit/test/widget/page_forward_transitions_test.dart

@@ -7,8 +7,6 @@ import 'package:flutter/animation.dart';
 import 'package:flutter/material.dart';
 import 'package:test/test.dart';
 
-import 'test_widgets.dart';
-
 class TestTransition extends TransitionComponent {
   TestTransition({
     Key key,

packages/unit/test/widget/set_state_2_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:flutter_test/flutter_test.dart';
+import 'package:flutter/widgets.dart';
+import 'package:test/test.dart';
+
+void main() {
+  test('setState() overbuild test', () {
+    testWidgets((WidgetTester tester) {
+      List<String> log = <String>[];
+      Builder inner = new Builder(
+        builder: (BuildContext context) {
+          log.add('inner');
+          return new Text('inner');
+        }
+      );
+      int value = 0;
+      tester.pumpWidget(new Builder(
+        builder: (BuildContext context) {
+          log.add('outer');
+          return new StatefulBuilder(
+            builder: (BuildContext context, StateSetter setState) {
+              log.add('stateful');
+              return new GestureDetector(
+                onTap: () {
+                  setState(() {
+                    value += 1;
+                  });
+                },
+                child: new Builder(
+                  builder: (BuildContext context) {
+                    log.add('middle $value');
+                    return inner;
+                  }
+                )
+              );
+            }
+          );
+        }
+      ));
+      log.add('---');
+      tester.tap(tester.findText('inner'));;
+      tester.pump();
+      log.add('---');
+      expect(log, equals(<String>[
+        'outer',
+        'stateful',
+        'middle 0',
+        'inner',
+        '---',
+        'stateful',
+        'middle 1',
+        '---',
+      ]));
+    });
+  });
+}

packages/unit/test/widget/snack_bar_test.dart

@@ -6,8 +6,6 @@ import 'package:flutter_test/flutter_test.dart';
 import 'package:flutter/material.dart';
 import 'package:test/test.dart';
 
-import 'test_widgets.dart';
-
 void main() {
   test('SnackBar control test', () {
     testWidgets((WidgetTester tester) {

packages/unit/test/widget/test_widgets.dart

@@ -59,10 +59,3 @@ void flipStatefulComponent(WidgetTester tester) {
   FlipComponentState state = stateElement.state;
   state.flip();
 }
-
-
-class Builder extends StatelessComponent {
-  Builder({ Key key, this.builder }) : super(key: key);
-  final WidgetBuilder builder;
-  Widget build(BuildContext context) => builder(context);
-}