25221 Revert “Call Dart plugin registrant if available” (cla: yes, waiting for tree to go green)

25222 started cleaning scenario tests before running them (cla: yes, waiting for tree to go green)

25226 Started waiting for the notifications locally before asserting side effects (cla: yes, platform-ios, waiting for tree to go green)

25241 [tracing] Add trace events for DartIsolate::HandleMessage (cla: yes)

25247 Highlight VSync support for embedders (cla: yes)

25248 Revert “Started waiting for the notifications locally before assertin… (cla: yes, platform-ios)

25252 Remove ignores that are not ignoring anything (cla: yes)