26688 [web] use isTrue/isFalse in all tests (cla: yes, platform-web)

26696 [web] test runner fixes: build, sourcemaps, Dart 1-isms (cla: yes, platform-web)

26709 [web] text_editing_test.dart nnbd; bump test dep (cla: yes, platform-web)

26712 [web] more null-safe tests (cla: yes, platform-web)

26719 [web] last batch of test null safety (cla: yes, waiting for tree to go green, platform-web)

26720 [web] migrate web_sdk to null safety (cla: yes, waiting for tree to go green, platform-web)