This feature can be used to implement password fields, where the user doesn't
want the text displayed on screen. Currently all the characters are hidden.
Eventualy we'll want to show the most recent character for some timeout.

Fixes #636

Add a default onEnter/onExit for EnterExitTransition

PopupMenu shouldn't use CustomPaint

We now default to the identity transition.

Bottom sheets shouldn't dismiss when flung up

Align is now powerful enough to do all the work we need to do here.

New methods for manipulating Navigators.

* NavigatorState.replaceRouteBefore()
* NavigatorState.removeRouteBefore()
* NavigatorState.popUntil()

Add a "replace()" API for the navigator.

This lets you dynamically replace one route with another.

Tabs scroll animation

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.

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.

Clean up code organization in flutter_tools

1) Moved basic utility code into base/ directory to make it clear which code
   doesn't depend on Flutter-specific knowldge.
2) Move the CommandRunner subclasses into a runner/ directory because these
   aren't commands themselves.

Remove build_sky_apk.dart

This script has been subsumed by `flutter apk`.

dev/update_packages.dart --upgrade

Add an --upgrade flag to the dev/update_packages.dart flag which runs
'pub upgrade' instead of 'pub get'. Tell people to use this when using
'flutter analyze' since 'pub get' doesn't have the same guarantee of
getting everything in sync.

Properly paint disabled switches

Also, refactor more common code into RenderToggleable and handle dark themes
more correctly.

Fixes #601

Add missing types for list literals

Improve Material selection controls

 - These controls now have proper radial reactions.
 - You can drag the switch.
 - The radio button animates properly.
 - There's a demo in the Material Gallery

Move RenderTogglable to material.dart

Move Material Gallery demos into a directory

Also, clean up the main.dart file to prepare for making dynamic theme changes.

Fixes #219

Operators needed to lerp a FractionalOffset.

List assets used by Stocks demo

Fix minor typos and style nits.

Turn the LocalHistoryRoute into a mixin.

Without this, you can't use AnimatedValue<FractionalOffset>, because its
lerp() function uses the operators.

Clean up the Navigator API again.