Element subclasses must call super in forgetChild

## Summary A recent global key duplication detection refactor now requires `Element` subclasses that override the `forgetChild` to call `super`. ## Context When encountering a global key duplication that will be cleaned up by an element rebuild later, we must not report global key duplication. Our previous implementation threw error as soon as duplication was detected, and didn't wait for the rebuild if the element with duplicated global key would have rebuilt. The new implementation keeps 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 an error immediately. As part of the refactoring, we implemented a mechanism to remove previous global key duplication in `forgetChild` if the rebuild had happened. This, however, requires all `Element` subclasses that override `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 the `@mustCallSuper` meta tag. All subclasses that override the method have to call super; otherwise, the analyzer shows a linting error and global key duplication detection might throw an unexpected error. ## Migration guide In the following example, an app's `Element` subclass overrides 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 This change landed in 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