Make default argument to Iterable.join be "".

sdk/lib/html/html_common/filtered_element_list.dart

 // Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of html_common;
 
 /**
  * An indexable collection of a node's descendants in the document tree,
  * filtered so that only elements are in the collection.
  */
@@ skipping 31 matching lines @@
     final len = this.length;
     if (newLength >= len) {
       return;
     } else if (newLength < 0) {
       throw new ArgumentError("Invalid list length");
     }
 
     removeRange(newLength, len - newLength);
   }
 
-  String join([String separator]) => _filtered.join(separator);
+  String join([String separator = ""]) => _filtered.join(separator);
 
   void add(Element value) {
     _childNodes.add(value);
   }
 
   void addAll(Iterable<Element> iterable) {
     for (Element element in iterable) {
       add(element);
     }
   }
@@ skipping 148 matching lines @@
   Element get single => _filtered.single;
 
   Element min([int compare(Element a, Element b)]) => _filtered.min(compare);
 
   Element max([int compare(Element a, Element b)]) => _filtered.max(compare);
 
   Map<int, Element> asMap() {
     return IterableMixinWorkaround.asMapList(this);
   }
 }