Version 0 of TabLabel, Tab, and TabBar components

There's is no support for animating the selected tab indicator, there isn't a TabNavigator container yet, overflow layout (tabs don't fit) isn't supported yet, etc.

R=abarth@chromium.org, ianh@google.com

Review URL: https://codereview.chromium.org/1205953002.

examples/widgets/tabs.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/theme/colors.dart';
+import 'package:sky/theme/typography.dart';
+import 'package:sky/widgets/basic.dart';
+import 'package:sky/widgets/material.dart';
+import 'package:sky/widgets/scaffold.dart';
+import 'package:sky/widgets/tabs.dart';
+import 'package:sky/widgets/tool_bar.dart';
+import 'package:sky/widgets/widget.dart';
+
+class TabbedNavigatorApp extends App {
+  static Iterable<String> items = const <String>["ONE", "TWO", "FREE", "FOUR"];
+  final List<int> navigatorSelections = new List<int>.filled(items.length, 0);
+
+  Widget buildTabNavigator(Iterable<TabLabel> labels, int navigatorIndex) {
+    TabBar tabBar = new TabBar(
+      labels: labels.toList(),
+      selectedIndex: navigatorSelections[navigatorIndex],
+      onChanged: (selectionIndex) {
+        setState(() { 
+          navigatorSelections[navigatorIndex] = selectionIndex;
+        });
+      }
+    );
+
+    return new Container(child: tabBar, margin: new EdgeDims.only(bottom: 16.0));
+  }
+
+  Widget build() {
+    Iterable<TabLabel> textLabels = items
+      .map((s) => new TabLabel(text: "ITEM " + s));
+
+    Iterable<TabLabel> iconLabels = items
+      .map((s) => new TabLabel(icon: 'action/search_white'));
+
+    Iterable<TabLabel> textAndIconLabels = items
+      .map((s) => new TabLabel(text: "ITEM " + s, icon: 'action/search_white'));
+
+    var navigatorIndex = 0;
+    Iterable<Widget> tabNavigators = [textLabels, iconLabels, textAndIconLabels]
+      .map((labels) => buildTabNavigator(labels, navigatorIndex++));
+
+    ToolBar toolbar = new ToolBar(
+      center: new Text('Tabbed Navigator', style: white.title),
+      backgroundColor: Blue[500]);
+
+    return new Scaffold(
+      toolbar: toolbar,
+      body: new Material(
+        child: new Center(child: new Block(tabNavigators.toList())),
+        color: Grey[500]
+      )
+    );
+  }
+}
+
+void main() {
+  runApp(new TabbedNavigatorApp());
+}