76747 [flutter_tools] remove mockito from mdns client test (cla: yes, tool, waiting for tree to go green)

76748 [flutter_tools] migrate async guard to null safety (cla: yes, tool)

76802 [flutter_tools] switch FakeCache to cache.test and NoopUsage to TestUsage (cla: yes, tool)

76806 [flutter_tools] simplify packages pass-through testing (cla: yes, tool, waiting for tree to go green)

76812 [flutter_tools] remove mock from terminal test (cla: yes, tool)

76813 [flutter_tools] remove mock from cmake project migrator (cla: yes, tool, waiting for tree to go green)

76821 [flutter_tools] replace most MockBuildSystem instances with TestBuildSystem (cla: yes, tool, waiting for tree to go green)