18620 Roll Fuchsia Mac SDK from hr-HZ… to 5yPfm… (cla: yes, waiting for tree to go green)

18622 Roll Dart SDK from e007545c42fd to 253022eb1b27 (5 revisions) (cla: yes, waiting for tree to go green)

18623 Roll Skia from fd9745eee98f to 3f479f3da7f8 (1 revision) (cla: yes, waiting for tree to go green)

18624 Fix copy/paste in the Linux embedding (cla: yes)

18625 Add tests & –unopt to build_fuchsia_artifacts (cla: yes, platform-fuchsia)