* 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.

Added animation to segmented control widget.

Put the nav bar padding inside the CustomMultiChildLayout for correct centering and allow custom padding (#19129)

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.

* Surface border parameter in CupertinoSliverNavigationBar (#18152)

* Add tests for CupertinoSliverNavigationBar border

* Remove unused keys, find by descendant

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.

This is a follow up on issue #17169 and the pull request #17298

This pull request adds the onChangeStart and onChangeEnd callbacks for CupertinoSlider. These are called when a user starts and ends a change respectively.

Pushing for @dcaraujo0872, the PR author.

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.

1. Make CupertinoSwitch use BoxShadow.toPaint() so that it respects
   the `debugDisableShadows` flag.
2. Increase blue radius on debug banner
3. Only stroke "synthetic shadow" borders if elevation is positive.

Semantics object support for edge triggered semantics 

* Let CupertinoTabScaffold's tab be programmatically selectable

* Re-use the tab bar's index instead

* review