Element subclasses must call super in forgetChild

## Summary A recent global key duplication detection refactor requires `Element` subclasses that override the `forgetChild` to call super. ## Context When we encounter a global key duplication which will be cleaned up by an element rebuild later, we must not report global key duplication. Our previous implementation throws error as soon as we detect such duplication and does not wait for the rebuild if the element with duplicated global key will rebuild. The current implementation will keep track all the global key duplications during a build cycle and only verify global key duplication at the end of the that cycle instead of throwing error immediately. As part of the refactoring, we implemented a mechanism to remove previous global key duplication in forgetChild if the rebuild has happened. This, however, requires all Element subclasses that override the forgetChild to call the super method. ## Description of change The `forgetChild` of abstract class `Element` has a base implementation to remove global key reservation, and it is enforced by `@mustCallSuper` meta tag. All the subclasses that override the method have to call super; otherwise, analyzer will show linting error and global key duplication detection might throw unexpected error. ## Migration guide In the following example, an app's `Element` subclass overrode the `forgetChild` method Code before migration: ```dart class CustomElement extends Element { @override void forgetChild(Element child) { ... } } ``` Code after migration: ```dart class CustomElement extends Element { @override void forgetChild(Element child) { ... super.forgetChild(child); } } ``` ## Timeline v1.14.7 ## References API documentation: * [`Element`] * [`forgetChild`] Relevant issues: * [Issue 43780] Relevant PRs: * [PR fix global key error] [`Element`]: https://api.flutter-io.cn/flutter/widgets/Element-class.html [`forgetChild`]: https://api.flutter-io.cn/flutter/widgets/Element/forgetChild.html [Issue 43780]: https://github.com/flutter/flutter/issues/43780 [PR fix global key error]: https://github.com/flutter/flutter/pull/46183