After flutter/engine#31720 the Dart Plugin Registrant can be executed manually. As long as it is only executed on background isolates it is idempotent. If someone however accidentally executes it from the root isolate, the registrant would be executed twice and it would be an error.

If we move the guard that makes it idempotent from dart:ui into the generated code, we can make sure that executing it form the root isolate multiple times is not an error.