Integrate Date Picker into material_gallery

Fix upload of composited scene with non-zero offsets

We weren't correctly offsetting the clips when uploading composited scenes. Now
we apply the proper offsets.

Allow run_mojo --android to specify location of flutter engine

Add package:flutter_test

This package contains WidgetTester, which is very useful when writing tests for
widgets.

Make it possible to run tests outside the Flutter repo

Add avatars to material chips

Fixes #464

This patch still requires a locally built engine. I'll remove the need for a
locally built engine in a future patch.

Fixes #278

Add basic support for material chips

http://www.google.com/design/spec/components/chips.html#chips-behavior

This patch adds support for deletable and non-deleteable chips, but doesn't yet
add support for contact chips. Also, demo the chips in a new Material Gallery
app that will let us demo our gallery of widgets in a single app.

more verbose error message

Avoid using transforms when simple offsets will do.

Rev engine, services, and sdk.

Clean up the term "modal" in the navigator

Provide a way to insert multiple overlays at once.

Assert: don't push a PageRoute over a PopupRoute.

"Fixes" #389.

Now we only use it for things related to ModalRoute and ModalBarrier.

(This is easy now that ephemeral routes are gone, so there's no other
kind of route to distinguish against.)

Fix analyzer warnings

Should make Travis green again. Also, add better logging to
dev/update_packages.dart.

Switch Travis to the stable Dart SDK

Hopefully this will make Travis green.

Build APKs without a local copy of the engine sources

Use embedder to fetch keyboard service

Update to engine 0.0.60

undo auto format

flutter build --engine-src-dir=<garbage> produces poor error message

This patch improves the error message.

Fixes #468

Route refactor

- Removed the concept of ephemeral routes.
- Renamed the two _MenuRoutes to _PopupMenuRoute and _DropDownRoute.
- Added type arguments in various places:
  - DropDownMenu
  - _DropDownRoute
  - _ModalBottomSheetRoute
  - PopupMenuItem
  - _PopupMenu
  - _PopupMenuRoute
- Made _ModalBottomSheetRoute, the two ex _MenuRoutes, and _DialogRoute
  all inherit from ModalRoute, via PopupRoute.
- Change "Dropdown" and "DropDown" to "DropDown" consistently.
- Made MaterialPageRoute inherit from PageRoute.
- Made ModalBarrier not create a box if it's always transparent.
- Exposed the Futures on TransitionRoutes.
- Fixed that menus were no longer dismissable by tapping the modal
  barrier.