18593 Roll Dart SDK from 65113fd73dec to 9e3b0289197d (2 revisions) (cla: yes, waiting for tree to go green)

18594 Roll Fuchsia Mac SDK from TSEOq… to hr-HZ… (cla: yes, waiting for tree to go green)

18595 Fixes FlutterEngine internal retain cycle (cla: yes)

18596 Fix wrong method name - copy/paste error from FlBinaryMessenger (cla: yes)

18597 Add tests for FlBinaryMessenger, FlBasicMessageChannel, FlMethodChannel (cla: yes)

18600 Revert “Update CanvasKit to 0.15.0 (#18570)” (cla: yes)