Fix analyzer errors in sample code (#10648)

packages/flutter/lib/foundation.dart

@@ -17,13 +17,14 @@ export 'package:meta/meta.dart' show
   required;
 
 // Examples can assume:
+// String _name;
 // bool _first;
 // bool _lights;
 // bool _visible;
 // double _volume;
-// dynamic _selection;
-// dynamic _last;
 // dynamic _calculation;
+// dynamic _last;
+// dynamic _selection;
 
 export 'src/foundation/assertions.dart';
 export 'src/foundation/basic_types.dart';

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

@@ -14,12 +14,24 @@ import 'constants.dart';
 /// The selected tab's index can be changed with [animateTo].
 ///
 /// A stateful widget that builds a [TabBar] or a [TabBarView] can create
-/// a TabController and share it directly.
+/// a [TabController] and share it directly.
+///
+/// When the [TabBar] and [TabBarView] don't have a convenient stateful
+/// ancestor, a [TabController] can be shared with the [DefaultTabController]
+/// inherited widget.
 ///
 /// ## Sample code
 ///
+/// This widget introduces a [Scaffold] with an [AppBar] and a [TabBar].
+///
 /// ```dart
-/// class _MyDemoState extends State<MyDemo> with SingleTickerProviderStateMixin {
+/// class MyTabbedPage extends StatefulWidget {
+///   const MyTabbedPage({ Key key }) : super(key: key);
+///   @override
+///   _MyTabbedPageState createState() => new _MyTabbedPageState();
+/// }
+///
+/// class _MyTabbedPageState extends State<MyTabbedPage> with SingleTickerProviderStateMixin {
 ///   final List<Tab> myTabs = <Tab>[
 ///     new Tab(text: 'LEFT'),
 ///     new Tab(text: 'RIGHT'),
@@ -58,10 +70,6 @@ import 'constants.dart';
 ///   }
 /// }
 /// ```
-///
-/// When the [TabBar] and [TabBarView] don't have a convenient stateful
-/// ancestor, a TabController can be shared with the [DefaultTabController]
-/// inherited widget.
 class TabController extends ChangeNotifier {
   /// Creates an object that manages the state required by [TabBar] and a [TabBarView].
   ///

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

@@ -143,7 +143,7 @@ class DefaultTextStyle extends InheritedWidget {
 ///
 /// ```dart
 /// new Text(
-///   'Hello, $name! How are you?',
+///   'Hello, $_name! How are you?',
 ///   textAlign: TextAlign.center,
 ///   overflow: TextOverflow.ellipsis,
 ///   style: new TextStyle(fontWeight: FontWeight.bold),