Switch default of `injectMembers()` to no-op and fix subclasses.

Another regression from #133 fixed here.  `injectMembers()` should no-op for generated code, so flip the default (so we don't have to generate them) and fix up subclasses.

On the way, switch some `UnsupportedOperationException`s to `AssertionError`s since these are code paths that should never be called.