36394 Add missing protobuf dependency (cla: yes, tool)

36396 Optimize the transformRect and transformPoint methods in matrix_utils. (cla: yes, framework)

36399 Added ThemeMode support to the Flutter Gallery (cla: yes, d: examples, team, team: gallery)

36402 Teach render objects to reuse engine layers (cla: yes, framework, severe: API break, severe: performance, ☸ platform-web)

36404 Make test back button label deterministic (cla: yes, team)

36409 Add searchFieldLabel to SearchDelegate in order to show a custom hint (cla: yes, f: material design, framework)

36410 Add plumbing for hello world startup test in devicelab (cla: yes, team)