20572 Convert format script to Dart (cla: yes)

20575 Update 1.21 engine to use Dart 2.10.0-7.2.beta (cla: yes)

20577 convert zircon and fuchsia to null-safety (cla: yes)

20579 rename “compositor”/”surface” to “canvaskit”/”html”; document conventions (cla: yes)

20580 Add null checks for CanvasElement attributes (cla: yes)

20581 Add android scenarios presubmit task. (cla: yes)

20594 Revert harfbuzz 2.7.0 (cla: yes)

20597 Revert “Add android scenarios presubmit task.” (cla: yes)