22265 Roll Dart SDK from 52783837369d to b43baaaa477d (723 revisions) (cla: yes, waiting for tree to go green)

22267 Make PlatformDispatcher.locale and locales return consistent values (cla: yes)

22268 Fix Linux handling of window exposure events (affects: desktop, cla: yes, needs tests, platform-linux)

22269 Roll Dart SDK from b43baaaa477d to a4fbabcd73dc (1 revision) (cla: yes, waiting for tree to go green)

22270 Fix code style issues in MacOS embedder (cla: yes, waiting for tree to go green)