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

* Revert "Revert "Make artifact downloading more robust for flaky networks" (#13995)"

This reverts commit 33d8a035.

* Use subdirectories to organize cached files

* Fix unauthorized import

renderObject.setFlatChildren expects List<RenderBox>, not List<RenderObject>.

...especially about TextDirection.

* Clean up output directory before copying over new files

* Use standard clean task command

* JSON.decode produces Map<String, dynamic> and List<dynamic>
objects. If a more tight type is required then object needs to
be converted explicitly (see dart-lang/sdk#31876);
* Completer<dynamic> produces Future<dynamic>. In Dart 2 it is
runtime error to assign Future<dynamic> to variable of type Future<T>;