Turn on `avoid_return_types_on_setters` and cleanup annotated setters.

Artifacts from local engine builds will be found based on the --local-engine flag

It's safe to remove the unneeded `void`s from setters since the blocking issues in the
`always_declare_return_types` lint have been fixed (https://github.com/dart-lang/linter/).  We can also safely flip the bit on  `avoid_return_types_on_setters`.

The context and child arguments are required.  We should assert that
they're non-null.

Fixes #3843

Host tools can be found in the artifact cache directory for the host platform.
If a developer wants to use a local engine build instead, then provide an
--engine-build flag that selects the specific engine build variant.

* be more lenient when looking for matching android sdk components

* use a null aware operator

Bump Dart SDK to 1.17.0-dev.3.0.

Keeping up with the Joneses!

Fix crash installing onto certain Android devices

* use vendored dart sdk; check the expected version of flutter

* search up directories looking for the flutter root

Turn on SDK summaries in flutter analyze.

The latest dev build has stable summaries so we should start using them.

(Also ensures that analysis options are propogating to the SDK analysis context --- see:  https://github.com/dart-lang/sdk/issues/26129.)

* send ext.flutter.exit

* listen for help restart, quit

* refactor into a separate class

Fixes #3835

Fixes #3835

Update flutter_tools for the "unoptimized" engine build flag and the new output directory naming scheme (#3832)

Upgrade gradle files for Android Studio 2.1

Fixes #3833

* Use mojo clipboard service for copy/paste toolbar.

BUG=https://github.com/flutter/flutter/issues/1567

Fixes #417

Remove gratuitous pub resolver from flutter analysis.