* controller, position and test

* Make controllers swappable

* WIP

* Create a ListWheelScrollPhysics

* Created picker and gallery demo and testing now

* Works. Ready to document and test.

* Document and add tests. Make the scroll controller more generic.

* minor cleanup

* review

* review

* fix tests

* stop using TransformLayers for now

This reduces the jank of bringing up the license screen further.
The remaining lost frame or two are caused by Dart itself, see:

   https://github.com/dart-lang/sdk/issues/31954
   https://github.com/dart-lang/sdk/issues/31959
   https://github.com/dart-lang/sdk/issues/31960

Fixes https://github.com/flutter/flutter/issues/5187

Framework side for https://github.com/flutter/engine/pull/4584 & https://github.com/flutter/engine/pull/4587.

Also rolls engine to 4c82c566edf394a5cfc237a266aea5bd37a6c172.

(this will be generated by flutter update-packages)

Adds unit tests for AndroidWorkflow.runLicenseManager().

AndroidSdk and AndroidStudio depends relatively heavily on filesystem
access to locate installed tools. Testing code that relies on either of
these classes benefits from a utility method to generate a
legitimate-looking Android SDK directory layout for testing.

flutter doctor --android-licenses requires Android sdkmanager version 26
or later. When sdkmanager is not available (SDKs earlier than v25),
direct users to SDK upgrade instructions. When it's installed but not
v26 or later, emit instructions to run sdkmanager --update.

Moves MockProcess, MockStdio and a few other useful mocks from
packages_test.dart to common/mocks.dart. These are useful for testing
code with interactive IO.

This adds a new constructor to MockProcess to provide additional flexibility.

Convenience getters for the the path to the Android SDK manager and the
currently installed version of the tool.

Pre-factoring to support better checks around the --android-licenses
command, which uses a feature of the SDK manager that is unsupported in
older versions of the tool.

Skia is trimming some of these transitive includes

contains the following change :
  - Roll of Dart version 8d9d68751a505426eb5f59a9d29f103fde6bd474 into the engine

* scrolling node eleminated

* remove second node

* fix ids

* Create ListWheelScrollView

* fix missing doc

* tweak docs a bit

* fix imports

* Add some tests

* review

* review and transform tests

* fix test

* repatch lost https://github.com/flutter/flutter/commit/977701cf695ec106a52dde9f3659a7b132826748

* review

* Review

* Default to strong for 'flutter test --preview-dart-2' mode

* Remove empty line

* Dont init to null

* Break up line to make it more readable

* Add missing type annotation

Fixes https://github.com/flutter/flutter/issues/13870.

* pass through preview-dart-2, strong, ipv6 in daemon mode

* ws change to kick the windows bot

* Move the splitting of licenses to an isolate

This improves (from horrific to terrible) the performance of the
license screen. It also introduces a feature in the foundation layer
to make using isolates for one-off computations easier.

The real problem that remains with this, though, is that transfering
data between isolates is a stop-the-world operation and can take an
absurd amount of time (far more than a few milliseconds), so we still
skip frames.

More work thus remains to be done.

* - Add profile instrumentation to the isolate compute() method
- Add profile instrumentation to the LicensePage
- Add profile instrumentation to the scheduleTask method
- Make scheduleTask support returning a value
- Make the license page builder logic use scheduled tasks so that it doesn't blow the frame budget

* Revert "Bump async, http, and vm_service_client packages (#14136)"

This reverts commit 7ffcce84.

* Revert "Exclude flutter doctor IDE validators in CI environments (#13816)"

This reverts commit 3258c546.

* Roll engine to pick up changes that allow 'flutter test --preview-dart-2'

* Pick up windows 'flutter test' fix

This is in attempt to resolve build failure on Windows.

* Make FadeTransition more efficient

* Update fade_transition_test.dart

* Update proxy_box.dart

* Fix the confusing-zero case with NestedScrollView.

* Update mock_canvas.dart

* Update tabs_demo.dart

* more tweaks

This rolls it to a green build.

This rolls it to a commit containing only the dart roll.