Make default argument to Iterable.join be "".

tools/dom/templates/html/impl/impl_Element.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;
 
 // TODO(jacobr): use _Lists.dart to remove some of the duplicated
 // functionality.
 class _ChildrenElementList extends ListBase<Element> {
   // Raw Element.
@@ skipping 45 matching lines @@
 
   bool any(bool f(Element element)) {
     for (Element element in this) {
       if (f(element)) {
         return true;
       }
     }
     return false;
   }
 
-  String join([String separator]) {
+  String join([String separator = ""]) {
     return _childElements.join(separator);
   }
 
   Iterable map(f(Element element)) {
     return _childElements.map(f);
   }
 
   Iterable<Element> where(bool f(Element element)) {
     return _childElements.where(f);
   }
@@ skipping 916 matching lines @@
   const ScrollAlignment._internal(this._value);
   toString() => 'ScrollAlignment.$_value';
 
   /// Attempt to align the element to the top of the scrollable area.
   static const TOP = const ScrollAlignment._internal('TOP');
   /// Attempt to center the element in the scrollable area.
   static const CENTER = const ScrollAlignment._internal('CENTER');
   /// Attempt to align the element to the bottom of the scrollable area.
   static const BOTTOM = const ScrollAlignment._internal('BOTTOM');
 }