Breaking change 63334: cancel inner type promotions on await and yield

[Paul Berry]

Issue 63334 proposes fixing a bug in control flow analysis that can lead to unsound analysis of nested functions involving a combination of type promotion with `await` or `yield`. In very rare cases, the fix will be more conservative than necessary, requiring users to add some extra type promotions.

If you have any thoughts or concerns, please leave your feedback on the GitHub issue: https://github.com/dart-lang/sdk/issues/63334.