25830 Fix a11y placeholder on desktop; auto-enable engine semantics (cla: yes, platform-web)

25832 Roll Dart SDK from 21aa7f8cbde8 to 90993fcb8554 (1 revision) (cla: yes, waiting for tree to go green)

25833 Roll Skia from 958a9395e6ca to fa06f102f13b (4 revisions) (cla: yes, waiting for tree to go green)

25834 Roll Dart SDK from 90993fcb8554 to b5ff349bffc8 (1 revision) (cla: yes, waiting for tree to go green)