Remove use of `@sealed` in DevTools

we are currently using `@sealed` because there is not an easy (or possible?) way to mock a `final` or `base` class. Some discussion here: https://github.com/dart-lang/site-www/issues/5143. We will not be able to move away from `@sealed` until a testing solution is found for the class modifier language feature, or unless we completely restructure our tests to use all fakes for final classes and no mocks (this is more overhead than the benefit is worth).