Changes:

- Move the SemanticsConfiguration update from RenderToggleable to each subclass, so that Switch can use toggled.
- Add image, liveRegion, toggled properties to Semantics, SemanticsConfiguration, SemanticsNode
- Added semanticsLabel and excludeFromSemantics to Image (the latter so that we avoid creating a semantics node)
- Added onDismiss semantics action which maps to the modal escape on iOS and dismiss action on Android.
- Added dismiss and liveRegion to snackbar widget
- Updated custom painter semantics to handle image, liveRegion, toggle
- Updated relevant tests to use correct flag/action

* re-enable lint unnecessary_const

* remove trailling whitespaces

* remove unnecessary const (after merge)

RenderAndroidView is responsible for sizing and displaying an embedded
Android view.
AndroidView is responsible for creating and disposing the Android view
and is using RenderAndroidView to display it.

Fixes https://github.com/flutter/flutter/issues/18971

* Add sample of multicolumn picker to Flutter Gallery
* Modify CupertinoPicker to be able to be rendered off-center.

This reverts commit cc1cf13e.

Instead, if the surface is 0x0 semantics will just generate one root semantics node with size of 0x0. The node will have no children because all children are invisible and (except for the root node with this change) invisible children are dropped.

* fixed segmented control golden test

* fixed segmented control golden test

* added cursorWidth, cursorRadius

* added default value for cursorWidth based on Apple specs

* test default cursorWidth

* removed cursorHeight stuff

* added functionality to keep cursor from blinking

* cursor width and radius is configurable + tests

* changed goldens repo version in goldens.version

* working version of configurable cursor (erased debugKeepCursorOn)

* minor changes

* docs

* changed textfield test that was failing due to new default cursorwidth

* added default value of cursorwidth in RenderEditable

* only run golden file tests on Mac

* cursor tests

* the tests are actually there now

* weak warning fixed

* switching to Linux

* changed default cursorWidth: 2.0 -> 1.0

* assorted changes, including changing text field test

* re-paint -> re-layout when changing cursorWidth

This should fix https://github.com/flutter/flutter/issues/18729

I'll add a gold test, and try to figure out how to remove the saveLayer without affecting the correctness later.

This should fix https://github.com/flutter/flutter/issues/18731 and I've added a unit test for it.

This fixes https://github.com/flutter/flutter/issues/18708

* Revert "manual rollback of of 18530 (#18531)"

This reverts commit 3f79f8cb.

* Fix the offset calculation

* Add a unit test

This is a follow up on https://github.com/flutter/engine/pull/5420
and https://github.com/flutter/flutter/issues/18057

As you can see from the diff, we also mistakenly saveLayer before
the clip at some places previously.

* Use RenderAnimatedOpacity within AnimatedOpacity widget (#15466)

* Fixed minor bug in RenderAnimatedOpacity

* Updated protected API for ImplicitlyAnimatedWidget

We have many hidden children because of iOS accessibility (see also #18410).

For example, in the tile screen of complex_layout app, there could be up to 20 children being painted without this PR. With this PR, only 12 get painted.

This should improve the performance of FL-53 by 40%.

fuchsia_tester.dart still assumes Dart 1. Previously, it ran tests directly
from source, flutter_platform.dart automatically runs a kernel compile when
operating in Dart 2 mode, but this assumes a functional Dart SDK is available
in the artifacts directly, and fuchsia_tester.dart mocks out the artifacts
directory with an empty temp dir.

Remaining work is:
1. Get the frontend server building as a dependency on Fuchsia.
2. Patch fuchsia_tester.dart to use a valid Dart SDK and frontend server.

This also reverts migration to Dart 2 typedef syntax.

This reverts commit 6c56bb24. (#18362)
This reverts commit 3daebd05. (#18316)

Now that Dart 1 is turned off, reapplying my change to turn on the prefer_generic_function_type_aliases analysis option, and fix all the typedefs to Dart 2 preferred syntax.

Also eliminated the unused analysis_options_repo.yaml file and turned on public_member_api_docs in analysys_options.yaml.

No logic changes, just changing the typedef syntax for all typedefs, and updating analysis options.

A bug in _RenderCustomClip was compaeing the type of oldClipper to
itself instead to the type of newClipper.

This was the root cause for the crash #14937 worked around.
This also reverts the workaround introduced in #14937.

Also, fixes "flutter run" for golden tests, and some docs.

Revert "Update typedef syntax to use Function notation and turn on lint for old notation. (#18035)" (#18041)

This reverts commit 32586020.