Rename Closure instance field type_arguments_ to instantiator_.

runtime/vm/parser.cc

Index: runtime/vm/parser.cc
diff --git a/runtime/vm/parser.cc b/runtime/vm/parser.cc
index 1afacf5b0017cb5c7c053202c0a0c4ff1e2e99f6..bb5f8d61bbfea7b6404fd9a8a8513f5ed7a3e3c9 100644
--- a/runtime/vm/parser.cc
+++ b/runtime/vm/parser.cc
@@ -4707,7 +4707,7 @@ void Parser::ParseClassDeclaration(const GrowableObjectArray& pending_classes,
     } else {
       // Not patching a class, but it has been found. This must be one of the
       // pre-registered classes from object.cc or a duplicate definition.
-      if (!(cls.is_prefinalized() ||
+      if (!(cls.is_prefinalized() || cls.IsClosureClass() ||
             RawObject::IsImplicitFieldClassId(cls.id()))) {
         ReportError(classname_pos, "class '%s' is already defined",
                     class_name.ToCString());