Add Iterable.fold (and Stream.fold) which replace `reduce`.

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);
   }
 
 $if DEFINE_LENGTH_AS_NUM_ITEMS
   // SVG Collections expose numberOfItems rather than length.
   int get length => numberOfItems;
 $endif
   dynamic reduce(dynamic initialValue, dynamic combine(dynamic, $E)) {
     return IterableMixinWorkaround.reduce(this, initialValue, combine);
   }
 
+  dynamic fold(dynamic initialValue, dynamic combine(dynamic, $E)) {
+    return IterableMixinWorkaround.fold(this, initialValue, combine);
+  }
+
 $if DEFINE_CONTAINS
   bool contains($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]) =>
       IterableMixinWorkaround.joinList(this, separator);
@@ skipping 158 matching lines @@
     return Lists.getRange(this, start, end, <$E>[]);
   }
 
   List<$E> getRange(int start, int rangeLength) =>
       sublist(start, start + rangeLength);
 
   Map<int, $E> asMap() =>
     IterableMixinWorkaround.asMapList(this);
 
   // -- end List<$E> mixins.