- Switches to async process execution, which now shows output as it happens instead of in chunks when the process completes
-  Now uses ProcessManager so that it may be mocked for the test.
- Adds in the download and install of mingit on Windows.
- Updated package dependencies because of added dependency on process package.

* Lock events during runApp's warm up frame

* move to scheduler binding

* let the one scheduleWarmUpFrame api always lock

* tweak test

`RenderSemanticsGestureHandler` is no longer a semantics boundary, which allows us to correctly mark disabled buttons as disabled without having their semantics size and semantics node id change unexpectedly.

Fixes https://github.com/flutter/flutter/issues/12589.
Fixes https://github.com/flutter/flutter/issues/11991.
See also https://github.com/flutter/flutter/issues/11993.

This change also required some refactoring to how we deal with `twoPaneSemantics` scrolling as it previously relied on `RenderSemanticsGestureHandler` being a semantics boundary. This should also make the underlying logic easier to understand.

In addition, the following minor changes are included in this PR:
* Removal of orphaned and unused `SemanticsConfiguration.isMergingDescendantsIntoOneNode`.
* Logic optimizations for `markNeedsSemanticsUpdate` .
* Fix for edge case where `MergeSemantics` failed to merge semantics.
* Use of emojis to better indicate leaf merging in the printed semantics tree.
* Better assert message for adding invisible child semantics nodes.
* Make some semantics tests robuster by not relying on creation order of SemanticsNode ids across test boundaries.

Fixes https://github.com/flutter/flutter/issues/13943.

* Revert "Change github readme links to be served by github (#13929)"

This reverts commit fd516d95.

* Revert " Rename SemanticsFlags to SemanticsFlag (#13994)"

This reverts commit 24e3f705.

* Revert "Make artifact downloading more robust for flaky networks (#13984)"

This reverts commit 4d37e03e.

to align with `SemanticsAction` and other enum-like types.

* First version

* Add a clarifying comment

* Add verify to ZipDecoder.decodeBytes call

* Review comments

* Need OS Error for missing files

* FileSystemException, not OSError.

(untested)

Fixes the last bits of #5759.

This reverts commit 57b4a525.

Fixes https://github.com/flutter/flutter/issues/13972

* Pin url_launcher to 1.0.3

* Commit results of 'bin/flutter update-packages --force-upgrade'

* Added TODO

* Explicitly report CFE output(when using --preview-dart-2)

* Fix test so it looks at errorText

* Fix expected text

Unsubscribe should remove RouteAware for all routes.

* Revert "Revert "Revert "Revert "Update Android sdkmanager for all platforms (#13912)" (#13922)" (#13923)" (#13935)"

This reverts commit f6fae1ce.

In this attempt we try to use SDK's with pre-accepted licenses.

* use SDK with pre-accepted licenses

* update readme

Fixes https://github.com/flutter/flutter/issues/11993

* it's fixed

* Add test for simulator log reader

Revert "Revert "Revert "Update Android sdkmanager for all platforms (#13912)" (#13922)" (#13923)" (#13935)

This reverts commit ad898ad0.

Back to the whiteboard w.r.t. Android SDK licenses.

* First version

* Prevent modification of .flutter during analytics test

* Pass in directory and override analyzer warning due to conditional import

* Review comments

This reverts commit 45289c08.