Address issues raised in PR #2047

Fix linter error regarding extra braces in string interpolation in devices.dart

Update status messages when unsupported devices are detected by the tools

check to see if the xcode eula has been signed

Remove ThemeData.primarySwatch

In the dark theme, there isn't really a primary swatch, so this API was
a sandtrap. Instead, be explicit about the colors we need for various
widgets in the theme.

Fixes #1277

Improve issue template

Fix link to bug-reports page. Also, hint at how to get the flutter
version.

Add TestGesture

This helper makes it easier to write correct tests that involve
gestures.

Fixes #1855

SnackBar should have a single optional action

Remove redundant material_ prefix

Animation status callbacks should be re-entrant

These files are already in the "material" folder. They don't need to be
prefixed with material again.

Fixes #702

add the ability to start and stop device polling

We should mutate the status of the AnimationController before calling
the status callbacks so that we're prepared to be re-entered.

Fixes #1911

The example in the spec with multiple actions is an anti-example.

Fixes #1876

Improve the style in the starter app

Fixes #2011

Remove download_dart_sdk.py

This script is not used anymore.

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