[Material] Update TabController to support dynamic Tabs

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

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'dart:math';
+
 import 'package:flutter/widgets.dart';
 
 import 'constants.dart';
@@ -289,6 +291,8 @@ class DefaultTabController extends StatefulWidget {
     this.initialIndex = 0,
     @required this.child,
   }) : assert(initialIndex != null),
+       assert(length >= 0),
+       assert(initialIndex >= 0 && initialIndex < length),
        super(key: key);
 
   /// The total number of tabs. Typically greater than one. Must match
@@ -359,7 +363,7 @@ class _DefaultTabControllerState extends State<DefaultTabController> with Single
       // automatically update the index of the controller to be the new last tab.
       int newIndex;
       if (_controller.index >= widget.length) {
-        newIndex = widget.length - 1;
+        newIndex = max(0, widget.length - 1);
       }
       _controller = _controller._copyWith(length: widget.length, index: newIndex);
     }

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

@@ -954,7 +954,7 @@ class _TabBarState extends State<TabBar> {
       if (_controller.length != widget.tabs.length) {
         throw FlutterError(
           'Controller\'s length property (${_controller.length}) does not match the \n'
-          'number of tab elements (${widget.tabs.length}) present in TabBar\'s tabs property.'
+          'number of tabs (${widget.tabs.length}) present in TabBar\'s tabs property.'
         );
       }
       return true;
@@ -1261,7 +1261,7 @@ class _TabBarViewState extends State<TabBarView> {
       if (_controller.length != widget.children.length) {
         throw FlutterError(
           'Controller\'s length property (${_controller.length}) does not match the \n'
-          'number of tab elements (${widget.children.length}) present in TabBar\'s tabs property.'
+          'number of tabs (${widget.children.length}) present in TabBar\'s tabs property.'
         );
       }
       return true;