Fix typo (used keys as a method instead of as a getter).

lib/html/dart2js/html_dart2js.dart

Index: lib/html/dart2js/html_dart2js.dart
diff --git a/lib/html/dart2js/html_dart2js.dart b/lib/html/dart2js/html_dart2js.dart
index 177d864d373dd20e6c9adf0ce4fd2703f3a6c73f..bfa451ad8402e6c8e23e3accaaa638dc1b76f443 100644
--- a/lib/html/dart2js/html_dart2js.dart
+++ b/lib/html/dart2js/html_dart2js.dart
@@ -11799,7 +11799,7 @@ class _DataAttributeMap implements AttributeMap {
 
   void clear() {
     // Needs to operate on a snapshot since we are mutating the collection.
-    for (String key in keys()) {
+    for (String key in keys) {
       remove(key);
     }
   }