67155 [null-safety] migrate app dependencies of flutter driver (a: accessibility, a: null-safety, a: tests, cla: yes, framework, team)

67171 [null-safety] add integration tests for sound null safety modes, add support for sound null safety in dart2js (a: null-safety, cla: yes, team, tool, waiting for tree to go green)

68157 [NNBD] Migrate some Widgets tests (a: null-safety, a: tests, cla: yes, framework, team, waiting for tree to go green)

69005 Fix null issue with dynamically updating from zero tabs for TabBar (a: null-safety, cla: yes, f: material design, framework, waiting for tree to go green)

69630 Update plugins dependencies for the Gallery test (a: null-safety, a: tests, cla: yes, team, tool)