Use `Function` as function type syntax.

pkg/compiler/lib/src/enqueue.dart

Index: pkg/compiler/lib/src/enqueue.dart
diff --git a/pkg/compiler/lib/src/enqueue.dart b/pkg/compiler/lib/src/enqueue.dart
index 12c5a42bb0382d4fb7fae6d83722c78a68889c3b..11adf9783ff4bd23a3cac89d071c65fe2be71de4 100644
--- a/pkg/compiler/lib/src/enqueue.dart
+++ b/pkg/compiler/lib/src/enqueue.dart
@@ -914,11 +914,9 @@ class _EnqueuerImpactVisitor implements WorldImpactVisitor {
   }
 }
 
-typedef void _DeferredActionFunction();
-
 class _DeferredAction {
   final Element element;
-  final _DeferredActionFunction action;
+  final void Function() action;
 
   _DeferredAction(this.element, this.action);
 }