Replace ShowPress with TapDown

Merge DisposableArenaMember and GestureArenaMember

Move velocity tracker to dart

Turns out there were no uses of GestureArenaMember other than those that
use DisposableArenaMember.

CustomMultiChildLayout delegates to a MultiChildLayoutDelegate. Widget children are specified by their index in the list of children.

Rename sky.yaml to flutter.yaml

Routes shouldn't be interactive when animating out

Somehow we missed this one file.

More flutter more better.

This patch introduces the notion of a ModalRoute that puts up a modal barrier
and makes the route invisible to hit testing when its animating out. This patch
also uses this mechanism in a number of places (including PageRoute). There are
still a few more cases to convert, but that's work for a future patch.

Fixes #1684

Move us to a version of sky_tools that doesn't hang when the tests crash.

The name of the parameter wasn't consistent with IconButton or DrawerItem, etc.

Fixes #1871

We were trying to do a hero animation from a page to itself, which doesn't make
any sense. Now we only render the "to" page offstage if it is different from
the "from" page and if its performance isn't already complete.

This test currently crashes the engine.
Because of a bug in the test harness, it then causes the tests to hang.
This disables the test for now, so that we can get test coverage again.

This patch makes it so drag-and-drop doesn't thrash the overlay list.

Switch to Navigator2

Removes sky_tool and shelldb, which are replaced by the flutter command line
tool.

Add PageStorage support to Navigator2

sky_engine is now at 0.0.45
sky_services is now at 0.0.45
flx is now at 0.0.4
flutter is now at 0.0.15
flutter_sprites is now at 0.0.12

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.