Rewrote CupertinoAlertDialog to look nearly identical to an alert dialog in iOS. This includes considerations for blur, translucent white background color, button sizing, gap dividers between buttons, and text scaling layout behavior. (#18381)

Allows the cupertino picker to be scroll infinitely by adding builder.

Adding CupertinoActionSheet, showCupertinoModalPopup

Add minOverscrollLength to make CupertinoScrollbar thumb correct size in long scroll views.

Segment width now determined by width of widest child + children widgets now centered within segments

It was causing problems rolling Flutter into Fuchsia

* 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