The forgetChild() method must call super

A recent global key duplication detection refactor now requires Element subclasses that override the forgetChild() to call super().

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 an error as soon as duplication was detected, and didn't wait for the rebuild if the element with the duplicated global key would have rebuilt.

The new implementation keeps track of all global key duplications during a build cycle, and only verifies 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.

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.

In the following example, an app's Element subclass overrides the forgetChild method.

Code before migration:

class CustomElement extends Element {
​
    @override
    void forgetChild(Element child) {
        ...
    }
}

Code after migration:

class CustomElement extends Element {
​
    @override
    void forgetChild(Element child) {
        ...
        super.forgetChild(child);
    }
}

Landed in version: 1.16.3
In stable release: 1.17

API documentation:

• Element
• forgetChild()

Relevant issues:

• Issue 43780

Relevant PRs:

• PR 43790: Fix global key error