Trivial tabs-related fixes (#9191)

Add some commentary around DefaultTabController.initialIndex.
Fix typo (nuull) in TabBar.
Make some asserts more detailed.

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

@@ -221,6 +221,8 @@ class DefaultTabController extends StatefulWidget {
   /// Creates a default tab controller for the given [child] widget.
   ///
   /// The [length] argument must be great than one.
+  ///
+  /// The [initialIndex] argument must not be null.
   DefaultTabController({
     Key key,
     @required this.length,
@@ -232,6 +234,8 @@ class DefaultTabController extends StatefulWidget {
   final int length;
 
   /// The initial index of the selected tab.
+  ///
+  /// Defaults to zero.
   final int initialIndex;
 
   /// This widget's child. Often a [Scaffold] whose [AppBar] includes a [TabBar].

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

@@ -353,7 +353,7 @@ class _DragAnimation extends Animation<double> with AnimationWithParentMixin<dou
 class TabBar extends StatefulWidget implements AppBarBottomWidget {
   /// Creates a material design tab bar.
   ///
-  /// The [tabs] argument must not be nuull and must have more than one widget.
+  /// The [tabs] argument must not be null and must have more than one widget.
   ///
   /// If a [TabController] is not provided, then there must be a
   /// [DefaultTabController] ancestor.
@@ -446,6 +446,18 @@ class _TabBarState extends State<TabBar> {
 
   void _updateTabController() {
     final TabController newController = config.controller ?? DefaultTabController.of(context);
+    assert(() {
+      if (newController == null) {
+        throw new FlutterError(
+          'No TabController for ${config.runtimeType}.\n'
+          'When creating a ${config.runtimeType}, you must either provide an explicit '
+          'TabController using the "controller" property, or you must ensure that there '
+          'is a DefaultTabController above the ${config.runtimeType}.\n'
+          'In this case, there was neither an explicit controller nor a default controller.'
+        );
+      }
+      return true;
+    });
     if (newController == _controller)
       return;
 
@@ -700,6 +712,18 @@ class _TabBarViewState extends State<TabBarView> {
 
   void _updateTabController() {
     final TabController newController = config.controller ?? DefaultTabController.of(context);
+    assert(() {
+      if (newController == null) {
+        throw new FlutterError(
+          'No TabController for ${config.runtimeType}.\n'
+          'When creating a ${config.runtimeType}, you must either provide an explicit '
+          'TabController using the "controller" property, or you must ensure that there '
+          'is a DefaultTabController above the ${config.runtimeType}.\n'
+          'In this case, there was neither an explicit controller nor a default controller.'
+        );
+      }
+      return true;
+    });
     if (newController == _controller)
       return;
 
@@ -876,7 +900,18 @@ class TabPageSelector extends StatelessWidget {
     final ColorTween selectedColor = new ColorTween(begin: Colors.transparent, end: color);
     final ColorTween previousColor = new ColorTween(begin: color, end: Colors.transparent);
     final TabController tabController = controller ?? DefaultTabController.of(context);
-    assert(tabController != null);
+    assert(() {
+      if (tabController == null) {
+        throw new FlutterError(
+          'No TabController for $runtimeType.\n'
+          'When creating a $runtimeType, you must either provide an explicit TabController '
+          'using the "controller" property, or you must ensure that there is a '
+          'DefaultTabController above the $runtimeType.\n'
+          'In this case, there was neither an explicit controller nor a default controller.'
+        );
+      }
+      return true;
+    });
     final Animation<double> animation = new CurvedAnimation(
       parent: tabController.animation,
       curve: Curves.fastOutSlowIn,

packages/flutter/test/material/app_bar_test.dart

@@ -9,7 +9,9 @@ import 'package:flutter_test/flutter_test.dart';
 
 Widget buildSliverAppBarApp({ bool floating, bool pinned, double expandedHeight }) {
   return new Scaffold(
-    body: new CustomScrollView(
+    body: new DefaultTabController(
+      length: 3,
+      child: new CustomScrollView(
         primary: true,
         slivers: <Widget>[
           new SliverAppBar(
@@ -29,6 +31,7 @@ Widget buildSliverAppBarApp({ bool floating, bool pinned, double expandedHeight
           ),
         ],
       ),
+    ),
   );
 }