75005 Remove “unnecessary” imports in material (cla: yes, f: cupertino, f: material design, framework, waiting for tree to go green)

75006 Remove “unnecessary” imports in cupertino tests (cla: yes, f: cupertino, framework, waiting for tree to go green)

75007 Remove “unnecessary” imports in foundation,gestures tests (cla: yes, framework, waiting for tree to go green)

75012 Remove “unnecessary” imports in flutter_tools tests (cla: yes, tool, waiting for tree to go green)

75022 Migrate driver tests in example/ to NNBD (cla: yes, d: examples, team, waiting for tree to go green)

75023 Migrate some benchmarks to NNBD (cla: yes, team, waiting for tree to go green)