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

67672 make router assert more strict (cla: yes, framework, waiting for tree to go green)

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

67675 Fix nullability for some routing related stuff (cla: yes, 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)

67683 make Router.of nullable (cla: yes, framework, waiting for tree to go green)