When a dirty node is not in the tree, we should not clear the dirty bit. #5588

Suppose you have two widgets that use global keys, let's call the A and B.

Suppose further that the first time you build the tree, B is a descendant of A.

Not suppose that at some point, the tree is rejiggered such that the following happens:

1. A, and its descendants including B, are removed from the tree.
2. A, and its descendants including B, are readded to the tree. This marks A and B dirty.
3. When A is built, it removes B from the tree, because it now has different children.
4. B is then processed as well, but it's not in the tree so it's dirty bit is cleared.
5. Finally, some other node deeper than either A or B puts B back in the tree.

At this point, B will be marked dirty, however, it is already in the dirty list, and this will fire an assertion.