* 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

Trying to get the build green. We will have to go through skipped tests as a more long-term task.

* Identify CHeckBox and RadioButton as such

* review feedback

* Escape path separator in regexps

* Update platform_helper.dart

* Update stack_trace_test.dart

* Update stack_trace_test.dart

* Dump exceptions live when they happen when testing tests with flutter run.

* Update binding.dart

* Document Android's SemanticsAction.tap behavior

Fixes https://github.com/flutter/flutter/issues/13219.

* review comment

Fixes https://github.com/flutter/flutter/issues/2877

Code that uses dataPatterns[loc] expects it to contain Map<String, String>, not Map<String, dynamic>.

Change gen_date_localizations.dart to generate datePatterns with a more precise type and regenerate date_localizations.dart.

* Set _imageInfo to null before adding listener to new ImageStream

Fixes an edge case where switching the ImageStream to an already completed ImageStream caused the Image widget to show nothing. ImageStreamCompleter calls the listener during addListener if their image is already loaded.

* Fix whitespace

* Add test to verify correct behavior

* Added test from mravn-google's PR

Seems like we don't yet run the flutter tests on Windows, but we're
close to being able to. This makes some minor changes to make that
more possible:

 - fix the stack parsing code to support Windows paths
 - fix the tests for the stack error handling code to handle Windows paths
 - skip some tests that rely on Ahem font metrics