51496 Fix expression evaluation test leaking flutter_tester processes. (cla: yes, tool)

51500 [flutter_tools] hide usage of package:mustache behind interface (cla: yes, tool)

51503 Validate empty pubspec plugin section (cla: yes, tool)

51505 ensure dart defines work on web and not web (cla: yes, framework)

51507 don’t do a version check when invoking flutter in test_test.dart (cla: yes, tool, waiting for tree to go green)

51508 Replace clean command test functions with test groups (cla: yes, tool)

51517 Add missing features to DefaultTextStyleTransition and AnimatedDefaultTextStyle (a: animation, a: typography, cla: yes, f: material design, framework, waiting for tree to go green)