Make default argument to Iterable.join be "".

tools/dom/templates/immutable_list_mixin.darttemplate

   // -- start List<$E> mixins.
   // $E is the element type.
 
   // From Iterable<$E>:
 
   Iterator<$E> get iterator {
     // Note: NodeLists are not fixed size. And most probably length shouldn't
     // be cached in both iterator _and_ forEach method. For now caching it
     // for consistency.
     return new FixedSizeListIterator<$E>(this);
@@ skipping 12 matching lines @@
   }
 
 $if DEFINE_CONTAINS
   bool contains($E element) => IterableMixinWorkaround.contains(this, element);
 $else
   // contains() defined by IDL.
 $endif
 
   void forEach(void f($E element)) => IterableMixinWorkaround.forEach(this, f);
 
-  String join([String separator]) =>
+  String join([String separator = ""]) =>
       IterableMixinWorkaround.joinList(this, separator);
 
   Iterable map(f($E element)) =>
       IterableMixinWorkaround.mapList(this, f);
 
   Iterable<$E> where(bool f($E element)) =>
       IterableMixinWorkaround.where(this, f);
 
   Iterable expand(Iterable f($E element)) =>
       IterableMixinWorkaround.expand(this, f);
@@ skipping 156 matching lines @@
     IterableMixinWorkaround.asMapList(this);
 
   String toString() {
     StringBuffer buffer = new StringBuffer('[');
     buffer.writeAll(this, ', ');
     buffer.write(']');
     return buffer.toString();
   }
 
   // -- end List<$E> mixins.