Reduce usage of .enclosingElement to get enclosing class.

lib/compiler/implementation/enqueue.dart

Index: lib/compiler/implementation/enqueue.dart
diff --git a/lib/compiler/implementation/enqueue.dart b/lib/compiler/implementation/enqueue.dart
index b2ff2a0985dfcb9c952f669c548a925e7eeb745a..36c050a676b5b23cc6dcc82ec96dcbb40d24673d 100644
--- a/lib/compiler/implementation/enqueue.dart
+++ b/lib/compiler/implementation/enqueue.dart
@@ -80,7 +80,7 @@ class Enqueuer {
     }
     if (!isResolutionQueue &&
         element.kind === ElementKind.GENERATIVE_CONSTRUCTOR) {
-      registerInstantiatedClass(element.enclosingElement);
+      registerInstantiatedClass(element.getEnclosingClass());
     }
     if (elements === null) {
       elements = getCachedElements(element);