Our test script is getting complicated enough that doing your own
testing with a custom engine has become tedious. Now you can just do:

```bash
(cd ~/dev/flutter/; FLUTTER_TEST_ARGS=--local-engine=host_debug_unopt dev/bots/test.sh)
```

...to run all your tests (assuming your flutter repo is in
`~/dev/flutter` and your engine in `~/dev/engine`).

* MultiTapGestureRecognizer previously would assert if there was no
   competition.
 * GestureArenaTeam would always select the first recongizer as the
   winner even if a later recognizer actually accepted the pointer
   sequence.
 * debugPrintStack would fail a type check if maxFrames was non-null.
 * FractionalOffset.lerp would throw a null-pointer exception if its
   second argument was null.

Also, add a number of tests for previously untested lines of code.

This is handy for debugging a test. Communication is logged to:
* `flutter_driver_commands_{x}.log`, where {x} is an integer, and
* (if requested by user) to stdout

fixes #7473

* Update to Dart SDK `1.22.0-dev.6.0`.

* Update SDK summary generation to create spec and strong summaries.

* Updated to dev.8.0.

* Fixed console out.

Clarifying that Dividers aren't restricted to usage in lists and Drawers. Maybe no one else is confused about this but I didn't realize it until I found them used in the calculator demo (https://github.com/flutter/flutter/blob/8ca4caa4406046a683c0e2d2a5cf6118fa56fef7/examples/flutter_gallery/lib/demo/calculator/home.dart)

The demo of the SimpleDialog had some useful code that should really be part of
the framework. This patch extracts it into a SimpleDialogOption widget.

Remove debugCheckHasScaffold because it is unused.

Also, add tests for InkWell, SimpleDialog, and other widgets.

If the size is empty, then _updatePaintData will produce an invalid transform
that yields a NaN canvas bounds rectangle

Fixes https://github.com/flutter/flutter/issues/7431

That test intentionally has analysis errors to check how the tools react
to that situation.

Previously, the Slider used a drag gesture recognizer to move the head
of the slider, but when the slider was in a vertical scroller, the
recognizer would wait until the user moved the pointer by enough pixels
to disambiguate between sliding the slider and scrolling the scroller.

That worked fine for actual drags, but the slider should also move when
the user taps the track. This patch introduces a tap recognizer to
handle that behavior.

To avoid the slider's drag and tap recognizers from competing with each
other in the arena, this patch introduces the notion of a
GestureArenaTeam, which lets several recognizers combine to form one
entry in the arena.  If that entry wins, the team picks the first of its
recognizers as the winner, avoiding latency.

Fixes #7454

Now IconThemeData.fallback is a factory constructor and IconThemeData.of() does
the work of computing the fallback for its clients.

Also, add tests for ImageIcon and ListItems.

Previously, the ticker would not stop when it was muted because it thought it
was already not ticking.

With frameSync enabled, flutter_driver actions will only be performed
when there are no pending frames in the app under test. This helps with
reducing flakiness.

The underlying TextPainter is not exposed, so this patch exposes some
useful metrics that clients might want to read.

fixes #7433

* relax timeouts for some service protocol calls

* remove 'note that' text

Previously, it was possible for the test harness to bail
and the test runner to complete before the platform plugin
triggered the collection of coverage data. This fixes the
race condition such that the pending coverage collection
task is recorded immediately after starting the process.

* Add missing IJ metadata

* Updates

* Add modules metadata

This widget isn't tested and isn't used by Fuchsia or internally by Google. The
Hero widget is a better way of generating hero animations.

This patch adds tests for radio buttons, the date picker, as well as a
number of other classes.

We were checking similar things in two places. Now we check everything in one
place.

Also, fix a few minor bugs found by the test.

* Add example IntelliJ metadata (flutter-intellij#607).

Ensures example projects open cleanly out-of-the-box w/ the Flutter Plugin.

See: https://github.com/flutter/flutter-intellij/issues/607.

* Fixed IML files.

Previously we asserted that the element was not in the dirty list, but there
are scenarios where the element can be in the dirty list already. This patch
makes markNeedsBuild handle those cases by simply resorting the dirty list.

Fixes #6119

The indexes passed to paragraph.getBoxesForRange are based on UTF-16 code
units.  If the caret is at a character that is encoded as multiple code units,
then the start and end indexes should be adjusted accordingly.

- output single-line JSON because iOS 9.x cannot output more than one line
- move the workaround for #7433 into LiveTestWidgetsFlutterBinding so that it works in all cases (e.g. animation_bench.dart on iOS)

* ignore exceptions from stdin.lineMode

* only catch IOException

It is now handled at a higher level for all commands.

Now that dart-lang/dartdoc#1236 is fixed.

This API was deprecated. Clients should use PlatformMessages instead.

As of Dart SDK 1.22.0-dev.5.0, `Process.exitCode` is no longer
mutable (that SDK version picks up https://github.com/dart-lang/sdk/commit/e5a16b1ca56414af3008afb8833660f95a9d06da).
This change allows the tools code to pass analysis in sdk versions both
before and after that change, to allow for analysis against both the host and
target sdks.

Also, make the exception handling for global key listeners slightly more
robust.

This reverts commit ef25a203 to unbreak the build.