- Fixed the bug where CupertinoRefreshControl doesn't work in the gallery demo on Android.
- Updated documentation on CupertinoRefreshControl
- Added comments to the gallery demo
- Added concrete examples to ScrollPhysics

* fix some formatting issues

* address review comments

* fix indent

* add trailing commas on list/map/parameters

* add trailing commas on Invocation with nb of arg>1

* add commas for widget containing widgets

* add trailing commas if instantiation contains trailing comma

* revert bad change

* enable lint unnecessary_new

* fix tests

* fix tests

* fix tests

Add a countdown timer picker as part of the Cupertino date picker.

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

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.

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

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