67591 Migrate more material tests (cla: yes, f: material design, framework, waiting for tree to go green)

67629 enable lint cast_nullable_to_non_nullable (a: tests, cla: yes, f: cupertino, f: material design, framework)

67656 Fix new analyzer rule failure (cla: yes, f: material design, framework)

67668 fix the tree (cla: yes, f: material design, framework)

67674 Migrate more material tests to NNBD (cla: yes, f: material design, framework)

67679 Search bar dark mode contrast (cla: yes, f: material design, framework, waiting for tree to go green)

67682 Final definite assignment (a: accessibility, cla: yes, f: cupertino, f: material design, framework, waiting for tree to go green)