Download sky_engine and sky_services from cloud storage

Downloading these packages from cloud storage simplifies our deployment
story because we can upload to cloud storage automatically from the
buildbot.

This patch also switches the responsibility for downloading the engine
artifacts to update_engine.sh. Centralizing this responsibility ensures
that the packages and the binaries are always in sync.

"flutter drive" command

Runs a test app and a driver test simultaneously, then stops the app.

Usage:

```
flutter drive --target=/path/to/test/app.dart
```

This command will look for `/path/to/test/app_test.dart` by
convention. We will expand into other ways of discovering tests in the
future.

tvOS, watchOS and old simulators are blacklisted in the tools

Gallery icons demo

iOS: In case Xcode is installed but the version is too old. Advise the user to update.

Enable background compilation

Always generate legal UTIs for project identifiers.

iOS: Misc tooling updates

In case the user changes the path of the Flutter project on disk, the Dart SDK or the Flutter repository itself, the old Xcode paths would be invalid. Now, we rewrite them when we build the project.

Use the project name to initialize the bundle name and identifier

Generate a font manifest JSON file from flutter.yaml

Tooltip Gallery demo

Remove ServiceRegistry

We don't need this anymore now that we have Mozart. The engine recently
stopped support it also.

Added GridTile

Simplify creating grid tiles with GridTileBar headers or footers.

make the '-o' option in create optional

Added GridTileBar, grid list gallery demo


Added GridTileBar - support for adding headers and footers to grid tiles.

Added a material gallery scrollable grid demo.

Fixes #1554
Fixes #1979

flutter_driver package

This commit contains:

- FlutterDriver API for e2e tests usable in conjunction with package:test
- FlutterDriverExtension to be enabled by the application in order to
  allow an external agent to connect to it and drive user interactions and
  probe into the element tree
- initial implementations of tap, findByValueKey and getText commands (to
  be expanded in future PRs)

Added a few operators to the Velocity class.

#1968

Teach flutter to run from a relative path