Don't bother storing 0 into the hash table if it's already there.

sdk/lib/_internal/compiler/implementation/lib/collection_patch.dart

Index: sdk/lib/_internal/compiler/implementation/lib/collection_patch.dart
diff --git a/sdk/lib/_internal/compiler/implementation/lib/collection_patch.dart b/sdk/lib/_internal/compiler/implementation/lib/collection_patch.dart
index b16f8696ed4efbb15fde8cf86c961930e6670c7c..da518282096a8daa0b3c8dbda198781faa727587 100644
--- a/sdk/lib/_internal/compiler/implementation/lib/collection_patch.dart
+++ b/sdk/lib/_internal/compiler/implementation/lib/collection_patch.dart
@@ -530,11 +530,10 @@ patch class HashSet<E> {
   }
 
   void _addHashTableEntry(var table, E element) {
-    if (!_hasTableEntry(table, element)) {
-      _length++;
-      _elements = null;
-    }
+    if (_hasTableEntry(table, element)) return;
     _setTableEntry(table, element, 0);
+    _length++;
+    _elements = null;
   }
 
   bool _removeHashTableEntry(var table, E element) {