55699 [flutter_tools] allow pulling performance data from assemble (cla: yes, tool)

55701 [flutter_tools] surface missing assets originating package (cla: yes, tool, waiting for tree to go green)

55704 [flutter_tools] ensure etag headers are ascii (cla: yes, tool)

55715 [flutter_tools] add –dart-define option for fuchsia build (cla: yes, tool, waiting for tree to go green)

55748 [ci] pin macOS image (cla: yes, waiting for tree to go green)

55749 Roll engine 2b94311a7764..4f888d66250e (10 commits) (cla: yes, engine, work in progress; do not review)