This reverts commit 2352672c.

Reland "Do not involve external_view_embedder in submit frame process if threads are not merged. #22275" (#22372)

https://dart.googlesource.com/sdk.git/+log/fe12b0536f42..a188781c9fc8

2020-11-06 dart-luci-ci-builder@dart-ci.iam.gserviceaccount.com Version 2.12.0-28.0.dev

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* update pubspecs to null-safe dependencies

* remove build_runner dep; upgrade quiver

* use analysis_options.yaml to control analysis scope

This appears to be blocking the Dart SDK roll.

This reverts commit 0d5f2e90.

Due to merge conflicts, this also reverts #22275:

Revert "Do not involve external_view_embedder in submit frame process if threads are not merged. (#22275)"

This reverts commit 016fbde1.

This (mostly) re-lands #21163, which was reverted in #22321

This switches from using onKeyDown to using dispatchKeyEvent on Android so that keys can be intercepted handled by the framework and not continue to be dispatched to other controls.

It also now intercepts key events sent to InputConnection.sendKeyEvent, as some IMEs do (e.g. the Hacker's Keyboard), and sends them to Flutter.

This fixes the problem where (for example) pressing TAB on a hardware keyboard sends the tab to both the text field and to the focus traversal system.

Note that we still can't intercept all keystrokes given to a soft keyboard, only those which the soft keyboard decides to send to InputConnection.sendKeyEvent, and system keys (like the back button) are only received if the IME doesn't eat them.

This allows us to not rely on surface methods for getting
the external view embedder.