Fix identical test in hash-set.

sdk/lib/core/set.dart

Index: sdk/lib/core/set.dart
diff --git a/sdk/lib/core/set.dart b/sdk/lib/core/set.dart
index b55ce76ce42014a43a02d7ca34a7099a1e295c35..7c1f2a748ce1b0790c3f7e99751b60b6fabcf33c 100644
--- a/sdk/lib/core/set.dart
+++ b/sdk/lib/core/set.dart
@@ -209,7 +209,7 @@ class _HashSetIterator<E> implements Iterator<E> {
 
   bool get hasNext {
     if (_nextValidIndex >= _entries.length) return false;
-    if (_entries[_nextValidIndex] == _HashMapImpl._DELETED_KEY) {
+    if (identical(_entries[_nextValidIndex], _HashMapImpl._DELETED_KEY)) {
       // This happens in case the set was modified in the meantime.
       // A modification on the set may make this iterator misbehave,
       // but we should never return the sentinel.