Make default argument to Iterable.join be "".

tools/dom/src/WrappedList.dart

 // Copyright (c) 2013, 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 dart.dom.html;
 
 /**
  * A list which just wraps another list, for either intercepting list calls or
  * retyping the list (for example, from List<A> to List<B> where B extends A).
  */
@@ skipping 17 matching lines @@
   void forEach(void f(E element)) { _list.forEach(f); }
 
   dynamic reduce(initialValue, combine(previousValue, E element)) =>
       _list.reduce(initialValue, combine);
 
   dynamic fold(initialValue, combine(previousValue, E element)) =>
       _list.fold(initialValue, combine);
 
   bool every(bool f(E element)) => _list.every(f);
 
-  String join([String separator]) => _list.join(separator);
+  String join([String separator = ""]) => _list.join(separator);
 
   bool any(bool f(E element)) => _list.any(f);
 
   List<E> toList({ bool growable: true }) =>
       new List.from(_list, growable: growable);
 
   Set<E> toSet() => _list.toSet();
 
   int get length => _list.length;
 
@@ skipping 98 matching lines @@
   Iterator _iterator;
 
   _WrappedIterator(this._iterator);
 
   bool moveNext() {
     return _iterator.moveNext();
   }
 
   E get current => _iterator.current;
 }