21278 Roll Dart SDK from 8c45e2e29cb4 to 8bd3017291e5 (1 revision) (cla: yes, waiting for tree to go green)

21279 Roll Fuchsia Mac SDK from i2PBGu2n-… to WsUbW2ZnA… (cla: yes, waiting for tree to go green)

21284 [web] dispatch browser event on flutter/service_worker channel (cla: yes)

21286 Fix iOS platform view’s mask view blocking touch events. (cla: yes, platform-ios)

21290 SecurityException: Permission Denial (cla: yes, platform-android)

21297 Make SkCanvas types @anonymous; reduce logging noise (cla: yes)

21298 implement decodeFromPixels (cla: yes, waiting for tree to go green)