Add support for modal, ephemeral, and contentless routes to Navigator2

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

simplify the ImageDecoder native code binding

This patch combines embedder.dart and shell.dart into one thing. We should now
handle a bunch of error cases better.

  * embedder.connectToApplication has moved to shell.connectToApplication,
    matching the rest of the mojo universe.
  * embedder.connecttoService has moved to shell.connnectToService (and merged
    with shell.requestService).
  * shell.requestService is now shell.connectToService, matching the rest of
    the mojo universe.
  * serviceRegistry has moved from embedder.serviceRegistry to a top-level
    getter.

Fixes #1803

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.

Adds API documentation to sprite physics

Add an initial implementation of Navigator2

Correct TabNavigatorWidth, tabs.dart example

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

sky_engine is now at 0.0.44
sky_services is now at 0.0.44
flx is now at 0.0.3
flutter is now at 0.0.14
flutter_sprites is now at 0.0.11

Fix crash when removing a card in card_collection

A tool to update all the pubspec.yaml versions when we're revving flutter.

Media query class for querying viewport dimensions

Some initial unit tests for package signing

MixedViewport didn't use the building:true flag when locking itself, so
when it caused a rebuild of its children, we assumed that nobody was
allowed to mark things dirty below the list, and things crashed when
Inherited people did in fact rebuild.

Also:
 - default offset for MixedViewport
 - don't bother rebuilding if the underlying RenderObject is going to
   rebuild anyway for some reason
 - better docs for the "items must have keys" assert
 - keep the FlipComponent stuff together in test_widgets.dart

Icon should use an enum rather than an int for size

Remove unused material edge definition

Material design icons are defined to work at specific sizes: 18, 24, 36, 48.
The current API doesn't reflect that and just takes a size int. If an invalid
size is chosen an error is printed to the console and no icon shows up.

Fixes #1816

The one we actually use is in material.dart.

The Rendering layer binding now multiplexes the dart:ui metrics change
notification so that multiple consumers can all listen to the change.

Fixes #1807

Add a position to the onTap callback

Fixes #1807

Improves physics debug drawing

- Change RouteArguments to pass the route's BuildContext rather than
  the Navigator. This caused the bulk of the examples/ and .../test/
  changes (those are mostly mechanical changes). It also meant I could
  simplify Navigator.of().

- Make initState() actually get called when the State's Element is in
  the tree, so you can use Foo.of() functions there. Added a test for
  this also.

- Provide a RouteWidget so that routes have a position in the Widget
  tree. The bulk of the route logic is still in a longer-lived Route
  object for now.

- Make Route.setState() only rebuild the actual route, not the whole
  navigator.

- Provided a Route.of().

- Provided a Route.writeState / Route.readState API that tries to
  identify the clients by their runtimeType, their key, and their
  ancestors keys, up to the nearest ancestor with a GlobalKey.

- Made scrollables hook into this API to track state. Added a test to
  make sure this works.

- Fix the debug output of GestureDetector and the hashCode of
  MixedViewport.

- Fixed ScrollableWidgetListState<T> to handle infinite lists.