25866 [web] Fix some regex issues in the sdk_rewriter (cla: yes, platform-web)

25867 Switch PlatformMessages to hold data in Mappings (platform-ios, platform-android, cla: yes, platform-macos, platform-fuchsia, embedder)

25868 A11y inspect (cla: yes, platform-fuchsia)

25869 Roll Dart SDK from 6397e8b91103 to ee8eb0a65efa (1 revision) (cla: yes, waiting for tree to go green)

25870 Roll Skia from 097263bb5089 to 5dfb3f40684b (1 revision) (cla: yes, waiting for tree to go green)