The TabBar's selection is now represented by a TabBarSelection object which encapsulates both the previous and currently selected indices and the Performance used to animate the selection indicator.
Added a TabBarView class which displays a tab's contents. It uses a shared TabBarSelection to stay in sync with a TabBar. The TabBarView scrolls in sync with the TabBar when the selection changes. Eventually it will allow one to fling the selection forward or backwards.
Added a tabBar property to ToolBar. Typically the corresponding TabBarView will be the body of the toolbar's Scaffold.
Removed TabNavigatorView and TabNavigator.
Added a widget gallery tabs demo page. Removed the old tabs demo.
