Fixes #464

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.

Give type arguments to routes.

Rename LabeledGlobalKey's argument to debugLabel

Fix analyzer warnings in updater package

These end up not actually being used, currently, because we don't have
generic methods, which you'd need for showDialog() and friends, and we
don't have any way to parameterise a class type at runtime, which you'd
need for MaterialApp routes, but it's a step in the right direction.

The old name was confusing.

Fixes #462

This class was renamed to MojoEventSubscription.

Be more specific about what `flutter build` does

first addition of _embedder.yaml

OverlayRoute.finished()

Rather than have delayed calls to super.didPop(), which raises my
eyebrow every time I see it, this provides a separate finished()
function to call, and uses the convention that if you want to call it
yourself, you just don't call super.didPop().

Elaborate HSVColor interface