67058 Migrate the tests of flutter_test to null-safety (a: tests, cla: yes, framework, waiting for tree to go green)

67083 [flutter] Update some tests in flutter/test (a: tests, cla: yes, f: material design, framework, waiting for tree to go green)

67100 Revert dispose images when done (a: tests, cla: yes, framework)

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

67166 migrate material to nullsafety (a: tests, cla: yes, f: cupertino, f: material design, framework, waiting for tree to go green)

67177 Reland dispose images when done (#67100) (a: tests, cla: yes, framework)