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

Add a basic material slider

Finish TimePicker

The slider in this patch works, but it's missing animations and many subtle
touches.

After this patch, TimePicker should work correctly.

Fixes #559

Opacity was dropping stuff on the floor.

Turns out we passed the wrong bounds to the PaintingContext constructor.

Fix, and assert that we don't do that again.

Rev the engine dependency to the latest engine.

fix analysis errors; add missing icon

Remove start_test

This test was a bad mock-based test that hasn't ever caught a bug but has
required a lot of maintenance. Currently its broken because its mock doesn't
match the real interface.

Add support for system-level routing

Now you can supply a route argument to flutter start:

  $ flutter start --route /foo

which will start the app at /foo instead of /.