Make default argument to Iterable.join be "".

tools/dom/templates/html/impl/impl_Node.darttemplate

 // Copyright (c) 2012, 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 $LIBRARYNAME;
 
 /**
  * Lazy implementation of the child nodes of an element that does not request
  * the actual child nodes of an element until strictly necessary greatly
  * improving performance for the typical cases where it is not required.
@@ skipping 137 matching lines @@
   dynamic reduce(dynamic initialValue,
       dynamic combine(dynamic previousValue, Node element)) {
     return IterableMixinWorkaround.reduce(this, initialValue, combine);
   }
 
   dynamic fold(dynamic initialValue,
       dynamic combine(dynamic previousValue, Node element)) {
     return IterableMixinWorkaround.fold(this, initialValue, combine);
   }
 
-  String join([String separator]) {
+  String join([String separator = ""]) {
     return IterableMixinWorkaround.joinList(this, separator);
   }
 
   Iterable map(f(Node element)) {
     return IterableMixinWorkaround.mapList(this, f);
   }
 
   Iterable<Node> where(bool f(Node element)) {
     return IterableMixinWorkaround.where(this, f);
   }
@@ skipping 256 matching lines @@
   }
 
   /** 
    * Print out a String representation of this Node.
    */
   String toString() => localName == null ? 
       (nodeValue == null ? super.toString() : nodeValue) : localName;
 
 $!MEMBERS
 }