15949 Fix data race in DartIsolateGroupData. (cla: yes)

15950 [web] Calling platform message callback after copy (cla: yes, platform-web)

15951 Gets the DPI for all awareness mode and older Windows versions (cla: yes, waiting for tree to go green)

15952 Do not reset the child isolate preparer if the isolate group data already has one (cla: yes)

15953 Ensure we export the various dart snapshot symbols on Fuchsia (cla: yes, waiting for tree to go green)

15954 Roll fuchsia/sdk/core/mac-amd64 from LAOYs… to 6_pZp… (autoroller: commit, cla: yes)

15955 Cache computed window.physicalSize in a FrameReference (cla: yes, waiting for tree to go green)