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.

* Inspector robustness fixes.
Avoid cases where setState was called on disposed object.
Avoid cases where the inspector wouldn't render as it was called
after the first frame rendered and no other frames were rendered.

* Ignore extraneous error messages outside of one skipped test

* Fix analysis error

When Flutter is vendored as a submodule, `.git` is a File whose contents
are a pointer to the Git directory. This change allows you to pin your
app to a certain version of Flutter via submodules.

Generate the "version" file from git tags.
Remove the old VERSION file and mentions of versions in pubspec.yaml files.
Replace the old update_versions.dart script with a new roll_dev.dart script.
Update "flutter channel".
Update "flutter upgrade", including making it transition from alpha to dev.
Update "flutter --version" and "flutter doctor".

Depends on https://github.com/flutter/engine/pull/4483

The picture may need to draw outside the bounds (specifically for shadows)

Fixes https://github.com/flutter/flutter/issues/12618