* Add a Scrollable builder, refactor ScrollableList, et al

* Add space between the dialog demo buttons

* removed vestigial code

According to the profile for the flow manual test, we're spending the vast
majority of our time recording timeline traces. This patch removes the timeline
traces, which greatly improves performance.

Also, optimize TransformLayer to avoid one matrix memcpy. I filed
https://github.com/google/vector_math.dart/issues/166 about an API that would
make this even faster.

This exposes the default throttling implementation, and an alternative
non-throttling implementation, of `debugPrint`.

* gracefully handle generating docs twice

* silently handle if directory doesn't exist

This makes it possible to substitute 'flutter run' for 'flutter test'
and actually watch a test run on a device.

For any test that depends on flutter_test:

1. Remove any import of 'package:test/test.dart'.

2. Replace `testWidgets('...', (WidgetTester tester) {`
      with `testWidgets('...', (WidgetTester tester) async {`

3. Add an "await" in front of calls to any of the following:
    * tap()
    * tapAt()
    * fling()
    * flingFrom()
    * scroll()
    * scrollAt()
    * pump()
    * pumpWidget()

4. Replace any calls to `tester.flushMicrotasks()` with calls to
   `await tester.idle()`.

There's a guarding API that you can use, if you have particularly
complicated tests, to get better error messages. Search for
TestAsyncUtils.

Remove the unimplemented `more` option from the copy/paste toolbar.

Punting this feature for the near time, so I'm removing the dead option
from the toolbar.

Fixes #3077

This reverts commit 55f9145e.

Turns out that this commit breaks apps that use the material library,
because of the _errorTextStyle DefaultTextStyle which has inherit:true.
Just setting it to false doesn't work, unfortunately, because then you
hit some sort of issue with merging that text style with others that
have inherit:true.

Use the FlutterError hotness instead.

Fixes #1290

To make it clear that this constructor requires an explicit style. Also
throw a descriptive error recommending the inherit constructor for
styles with the inherit bit set.

Fixes #3842

* put an index at root of API docs bucket

* ignore all of dev/docs/doc dir, fix a bug, remove private names from doc gen script

* tweaks from review

* allow per-command analytics suppression

* rev the usage package dep

* send debug events for flutter lifecycle events

* remove the route event

* Added performance instrumentation for compositing addToScene() calls.

* Switched to non-callback versions of the Timeline API.

* Fixed Dart formatting nit.

* handle an exit code from android n

* review comments

Re-enable `one_member_abstracts` lint (#3608).

* updates from the flutter gallery rename

* rename ids to io.flutter.gallery

* add a webmaster tools verification file

* also put the verification file at the root of our bucket

* add comment for when we can simplify this script

- Handles appear with tap or long press.
- Toolbar appears with long press on text, or tap on handle.
- Correct toolbar items shown depending on context.

* shrink analytics message to < 80 chars

* catch line ending

* rename service_protocol.dart to protocol_discovery.dart

* add a wrapper around the obs. protocol

* use json-rpc in run

* consolidate obs. code; implement flutter run --benchmark

* review comments

Fix Flutter Gallery to use title case

@HansMuller

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