Fix a type warning

R=ahe@google.com

Review URL: https://chromereviews.googleplex.com/546987013 .

lib/transformations/closure/info.dart

diff --git a/lib/transformations/closure/info.dart b/lib/transformations/closure/info.dart
index c0231f1..64b98b5 100644
--- a/lib/transformations/closure/info.dart
+++ b/lib/transformations/closure/info.dart
@@ -105,8 +105,9 @@
       // Ignore the `length` method of [File] subclasses for now, as they
       // will force us to rename the `length` getter (kernel issue #43).
       // TODO(ahe): remove this condition.
+      Class parent = node.parent;
       if (node.name.name != "length" ||
-          node.parent.enclosingLibrary.importUri.toString() != "dart:io") {
+          parent.enclosingLibrary.importUri.toString() != "dart:io") {
         declaredInstanceMethodNames.add(node.name);
       }
     }