Bottom sheets shouldn't dismiss when flung up

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.

* Split didPush() into didPush() and install(), so that we can install
  the overlays without triggering the push logic. This will be used in a
  subsequent patch to implement route replacement.

* Split didPop() into didPop() and dispose(), so that we can remove
  overlays without triggering the pop logic. Also for a subsequent patch
  that implements replacement.

* Clean up _navigator on the routes when the Navigator itself is
  disposed.

* Drop the forwarding logic on willPushNext() -- now didPushNext() --
  and didPopNext(), since we no longer have StateRoutes to get in the
  way.

* Implement isCurrent more broadly and without having to keep track of
  state.

* Provide some toString()s on NamedRouteSettings and ModalRoutes.

* Make OverlayState.initState() use the insertAl functionality.

* Make OverlayRoute.builders abstract since that way you'll catch when
  you forget to do it. If you don't want overlays, don't inherit from
  this class.

* Made handleStatusChanged() on TransitionRoute public so that it can be
  overridden by subclasses.

Animate slider radial reaction

Also, have the slider draw different in its disabled state.

print logging timestamps to profile app launch

allow logs to work from any location