Previously MaterialApp was responsible for ending the activity when the
back stack was empty. However, this behavior is more general than
material. This patch moves the behavior to FlutterWidgetBinding, which
has a global view of all the binding observers.

Fixes #1086

Now with fewer lists.

Users of MaterialApp can provide an onLocaleChanged handler that will be
called to asynchronously fetch locale-specific data.  MaterialApp will
then instantiate a LocaleQuery that supplies the locale data to its
descendants.

Also:
 - minor code reindents in places.
 - reset the widget tree between tests.
 - once you generate a route, don't let its builder change
   (previously it would keep changing as the routes table changed).
 - revert the stocks app toolbar-fading-on-forward-transition thing.

To make it easier to avoid pushing twice in one frame, provide a
transaction mechanism for the navigator.

Now you can supply a route argument to flutter start:

  $ flutter start --route /foo

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

PageRoute is now MaterialPageRoute.

This also changes the following:

- Now the HeroController is a Navigator observer, rather than a feature
  of HeroPageRoutes, which are gone. This means heroes can work between
  any kind of ModalRoute now.

- ModalPageRoute is moved from modal_barrier.dart to routes.dart.

- It allows routes to opt-out of their modal barrier being a shortcut to
  popping the route.

- Features of PageRoute that aren't Material-specific get promoted to
  ModalRoute features: storage, the subtree key, offstageness...

The AnimatedModalBarrier is still a ModalRoute feature.

These types belong at the MaterialApp level.

Fixes #1969

- runApp(new MaterialApp()) was crashing long after the constructor. Now
  it asserts in a more useful location.
- remove the default name for NamedRouteSettings. It was unused anyway.

In this approach, the hero support is layered on top of the basic navigator
functionality.

Routes can now create a list of widgets, which can be empty (in the case of
contentless routes) or have multiple entries (in the case of modal routes).

Ephemeral routes are handled by keeping a separate list of modal and ephemeral
routes.

This navigator can handle simple page navigation. I'll add more features in
subsequent CLs.

Now AssetImage will look in the default bundle if you don't supply an explicit
bundle.

Fixes #680

Assert at build time that PageRoute route builders do not return null
widget trees.

Also very minor new code comments and code reorg to help make the heroes
patch easier to review.

(These are changes that are unrelated to Heroes but that were part of
the Heroes patch.)

The goal is to follow the guidelines in
https://github.com/flutter/engine/blob/master/sky/specs/style-guide.md#packages

Fixes #1638

MaterialApp assumes that you're using material design.

Also move radial reaction and fix imports for stats box.

These are now part of material.dart.

In the vast majority of cases, folks should be interacting with the Widget
rather than its State.

Fixes #267

Fixes #1372

This is probably a sign that you're using fn2 still.

This argument isn't needed anymore now that State has a getter for context.

Make it possible for named routes to be generated on the fly.

To demonstrate this, you can now long-press a stock to open it.

Next steps:

 - transitions between (named) states that follow full material logic,
   e.g. in the case of the stock row to stock page transition, expanding
   the row into a raised sheet of material and expanding it to fit the
   screen, leaving the toolbar in place but cross-fading the old
   contents to the new contents.

 - more information in the stock view.

While I was here I also made Material have an opinion about default text
style, so if you forget to set one, it just uses body1.

Also, fixed bugs introduced recently that made RouteState and MenuRoute
not work properly.

Introduce a Draggable class that wraps all the logic of dragging
something and dropping it on a DragTarget.

Update examples/widgets/drag_and_drop.dart accordingly.

Make the performance/transition part of routes optional.