It was causing problems rolling Flutter into Fuchsia

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)

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

This reverts commit cc1cf13e.

All slivers must start with Sliver (or CupertinoSliver).
All widgets must have a key argument.

Also, some minor tweaks here and there to style and docs.

This appears to trigger an error during AOT snapshot of the gallery as
follows:

```
stdout: [   +6 ms] Building AOT snapshot in release mode (ios-release)...
stdout: [  +15 ms] Extra gen_snapshot options: [--print_snapshot_sizes]
stdout: [   +7 ms] /usr/bin/arch -x86_64 /Users/flutter/.cocoon/flutter/bin/cache/artifacts/engine/ios-release/gen_snapshot --await_is_keyword --causal_async_stacks --packages=.packages --url_mapping=dart:ui,/Users/flutter/.cocoon/flutter/bin/cache/pkg/sky_engine/lib/ui/ui.dart --url_mapping=dart:vmservice_io,/Users/flutter/.cocoon/flutter/bin/cache/pkg/sky_engine/sdk_ext/vmservice_io.dart --embedder_entry_points_manifest=/Users/flutter/.cocoon/flutter/bin/cache/artifacts/engine/ios-release/dart_vm_entry_points.txt --embedder_entry_points_manifest=/Users/flutter/.cocoon/flutter/bin/cache/artifacts/engine/ios-release/dart_io_entries.txt --print_snapshot_sizes --snapshot_kind=app-aot-assembly --assembly=build/aot/arm64/snapshot_assembly.S /Users/flutter/.cocoon/flutter/examples/flutter_gallery/lib/main.dart
stderr: [+9324 ms] Error: 'file:///Users/flutter/.cocoon/flutter/examples/flutter_gallery/lib/demo/cupertino/cupertino_segmented_control_demo.dart': error: line 19 pos 1: circular dependency for function _CupertinoSegmentedControlDemoState
stderr: [        ] class _CupertinoSegmentedControlDemoState extends State {
stderr: [        ] ^
stderr: [  +33 ms] Dart snapshot generator failed with exit code 254
stderr: [        ] Snapshotting (IOSArch.arm64) exited with non-zero exit code: 254
stdout: [        ] Built to build/aot/.
stdout: [  +10 ms] "flutter aot" took 9,485ms.
```

This reverts commit 9d49ee3b.

* Adding segmented control to gallery

Move the notch computation from the FAB to the BAB.

The notch in the BAB (bottom action bar) for the FAB (floating action button) was previously kept as part of the FAB's implementation. This was done to keep the shape of the FAB and the shape of the notch coupled.
That approach resulted in a somewhat complex and 'non Fluttery' mechanism for propagating the notch computation from the FAB to the BAB.

This CL uncouples the FAB and the notch computation.
With the new API the BAB computes its overall shape including the notch using a NotchedShape delegate.

This includes multiple breaking changes:
  * Scaffold.setFloatingActionButtonNotchFor is deleted.
  * The ComputeNotch type is deleted.
  * The hasNotch property of BottomAppBar is deleted.
  * The notchMargin property of FloatingActionButton is deleted.

Quick migration guide from the previous API:

| Previous API | New API |
| ------------------|-------------|
| BottomAppBar(hasNotch: false) | BottomAppBar() |
| Using a FloatingActionButton with: BottomAppBar() / BottomAppBar(hasNotch: true) | BottomAppBar(shape: CircularNotchedRectangle()) |
| Scaffold.setFloatingActionButtonNotchFor(..) | No longer supported |

This adds an HSLColor class which uses a perceptual color space based upon human perception of colored light (as opposed to HSV, which is based on pigment colors).

You can see the difference in the color spaces here: https://en.wikipedia.org/wiki/HSL_and_HSV

I also added a "within" matcher for both HSLColor and HSVColor that will check if the (floating point) color components are within a certain error.

And tests.

When setState() calls occur asynchronously, it's possible that the
Futures on which they're waiting don't complete until the widget is
offscreen. To avoid this, we check the widget is mounted before calling
setState().

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.

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

This reverts commit 32586020.

Fixes #18028

Just changes typedef declarations, no logic changes.

Our style guide says the k's are not necessary, and it seems like a good idea to make all the code be consistent on this.

Only naming changes to private vars: no logic changes.

This replaces abandoned PR #14824 by @ekbiker, and gives it some love.

I'm moving the assets in the assets-for-api-docs repo to a slightly different location to help with organization in that repo, so this PR points the doc URLs to the new location. The old assets won't be removed until this PR makes its way to the API docs website.

No documentation or code changes here, other than changing doc image URLs.