* Fix shadows in NestedScrollView

The previous logic (and especially the previous test) wasn't quite right.

* Update nested_scroll_view.dart

This widget evolved over time by multiple people, I'm just trying to explain
in a more direct way what the widget is currently doing.

When running with --preview-dart-2, host variant of the engine is
required. Simulator builds, like device builds, should resolve
host_RUNTIMEVARIANT_OPTVARIANT, not host_RUNTIMEVARIANT_sim_OPTVARIANT.

Fixes https://github.com/flutter/flutter/issues/14232

(I haven't tested this with older versions of git, I'm just going on
what @jason-simmons wrote in the bug.)

* Point to master API docs

I _think_ this is the right place, since people contributing and working on master might care about where the API docs are for master branch.

* Update CONTRIBUTING.md

update from review comment

* Roll engine to 6921873c71e700235c0f68f0359be2332f93c8bc

Also some trivial improvements to style.

Roll engine to 7c34dfafc9acece1a9438f206bfbb0a9bedba3bf

* Unpin package:test and upgrade packages

* Update packages/flutter/test/foundation/stack_trace_test.dart

* Also add packages/flutter_tools/test/data/asset_test/font/.dartignore to ensure that update-packages --force-upgrade does not crash.

The code was attempting to delete temporaryDirectory while it should have been deleting tempBundleDirectory

* 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