Factor BackButton out of AppBar (#8491)

This widget is useful on its own. This patch factors it out of AppBar so
folks can use it separately.

Fixes #8489

packages/flutter/lib/material.dart

@@ -15,6 +15,7 @@ export 'src/material/about.dart';
 export 'src/material/app.dart';
 export 'src/material/app_bar.dart';
 export 'src/material/arc.dart';
+export 'src/material/back_button.dart';
 export 'src/material/bottom_navigation_bar.dart';
 export 'src/material/bottom_sheet.dart';
 export 'src/material/button.dart';

packages/flutter/lib/src/material/app_bar.dart

@@ -8,6 +8,7 @@ import 'package:flutter/foundation.dart';
 import 'package:flutter/services.dart';
 import 'package:flutter/widgets.dart';
 
+import 'back_button.dart';
 import 'constants.dart';
 import 'flexible_space_bar.dart';
 import 'icon.dart';
@@ -188,7 +189,7 @@ class AppBar extends StatefulWidget {
   /// example, if the [AppBar] is in a [Scaffold] that also has a [Drawer], the
   /// [Scaffold] will fill this widget with an [IconButton] that opens the
   /// drawer. If there's no [Drawer] and the parent [Navigator] can go back, the
-  /// [AppBar] will use an [IconButton] that calls [Navigator.pop].
+  /// [AppBar] will use a [BackButton] that calls [Navigator.maybePop].
   final Widget leading;
 
   /// The primary widget displayed in the appbar.
@@ -345,10 +346,6 @@ class _AppBarState extends State<AppBar> {
     Scaffold.of(context).openDrawer();
   }
 
-  void _handleBackButton() {
-    Navigator.of(context).maybePop();
-  }
-
   @override
   Widget build(BuildContext context) {
     final ThemeData themeData = Theme.of(context);
@@ -383,24 +380,8 @@ class _AppBarState extends State<AppBar> {
           tooltip: 'Open navigation menu' // TODO(ianh): Figure out how to localize this string
         );
       } else {
-        if (_canPop) {
-          IconData backIcon;
-          switch (Theme.of(context).platform) {
-            case TargetPlatform.android:
-            case TargetPlatform.fuchsia:
-              backIcon = Icons.arrow_back;
-              break;
-            case TargetPlatform.iOS:
-              backIcon = Icons.arrow_back_ios;
-              break;
-          }
-          assert(backIcon != null);
-          leading = new IconButton(
-            icon: new Icon(backIcon),
-            onPressed: _handleBackButton,
-            tooltip: 'Back' // TODO(ianh): Figure out how to localize this string
-          );
-        }
+        if (_canPop)
+          leading = const BackButton();
       }
     }
     if (leading != null) {

packages/flutter/lib/src/material/back_button.dart

@@ -0,0 +1,60 @@
+// Copyright 2017 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/widgets.dart';
+
+import 'theme.dart';
+import 'icon_button.dart';
+import 'icon.dart';
+import 'icons.dart';
+
+/// A material design back button.
+///
+/// A [BackButton] is an [IconButton] with a "back" icon appropriate for the
+/// current [TargetPlatform]. When pressed, the back button calls
+/// [Navigator.maybePop] to return to the previous route.
+///
+/// When deciding to display a [BackButton], consider using
+/// `ModalRoute.of(context)?.canPop` to check whether the current route can be
+/// popped. If that value is false (e.g., because the current route is the
+/// initial route), the [BackButton] will not have any effect when pressed,
+/// which could frustrate the user.
+///
+/// Requires one of its ancestors to be a [Material] widget.
+///
+/// See also:
+///
+///  * [AppBar], which automatically uses a [BackButton] in its
+///    [AppBar.leading] slot when appropriate.
+///  * [IconButton], which is a more general widget for creating buttons with
+///    icons.
+class BackButton extends StatelessWidget {
+  /// Creates an [IconButton] with the appropriate "back" icon for the current
+  /// target platform.
+  const BackButton({ Key key }) : super(key: key);
+
+  /// Returns tha appropriate "back" icon for the given `platform`.
+  static IconData getIconData(TargetPlatform platform) {
+    switch (platform) {
+      case TargetPlatform.android:
+      case TargetPlatform.fuchsia:
+        return Icons.arrow_back;
+      case TargetPlatform.iOS:
+        return Icons.arrow_back_ios;
+    }
+    assert(false);
+    return null;
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return new IconButton(
+      icon: new Icon(getIconData(Theme.of(context).platform)),
+      tooltip: 'Back', // TODO(ianh): Figure out how to localize this string
+      onPressed: () {
+        Navigator.of(context).maybePop();
+      },
+    );
+  }
+}

packages/flutter/test/material/back_button_test.dart

@@ -0,0 +1,37 @@
+// Copyright 2016 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/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  testWidgets('BackButton control test', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      new MaterialApp(
+        home: new Material(child: new Text('Home')),
+        routes: <String, WidgetBuilder>{
+          '/next': (BuildContext context) {
+            return new Material(
+              child: new Center(
+                child: const BackButton(),
+              )
+            );
+          },
+        }
+      )
+    );
+
+    tester.state<NavigatorState>(find.byType(Navigator)).pushNamed('/next');
+
+    await tester.pump();
+    await tester.pumpUntilNoTransientCallbacks();
+
+    await tester.tap(find.byType(BackButton));
+
+    await tester.pump();
+    await tester.pumpUntilNoTransientCallbacks();
+
+    expect(find.text('Home'), findsOneWidget);
+  });
+}