Stop using FunctionTypeAliasElement in nnbd_migration.

Change-Id: I3dfc310d28f2f3127e61302dc31ab11ee55d51d7
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/182661
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Konstantin Shcheglov <scheglov@google.com>
diff --git a/pkg/nnbd_migration/lib/src/variables.dart b/pkg/nnbd_migration/lib/src/variables.dart
index 84e0ea0..0362d62 100644
--- a/pkg/nnbd_migration/lib/src/variables.dart
+++ b/pkg/nnbd_migration/lib/src/variables.dart
@@ -395,7 +395,7 @@
       // case) `Function(T)`. Without this we would get `Function<T>(T)` which
       // is incorrect. This is a known issue with `.type` on typedefs in the
       // analyzer.
-      element = (element as FunctionTypeAliasElement).aliasedElement;
+      element = (element as TypeAliasElement).aliasedElement;
     }
 
     var target = NullabilityNodeTarget.element(element, _getLineInfo);