Make List.skip, List.take and List.reversed return Iterables, not Lists.

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 89 matching lines @@
 $endif
 
 $if DEFINE_CLEAR
   void clear() {
     throw new UnsupportedError("Cannot clear immutable List.");
   }
 $else
   // clear() defined by IDL.
 $endif
 
-  List<$E> get reversed {
+  Iterable<$E> get reversed {
     return IterableMixinWorkaround.reversedList(this);
   }
 
   void sort([int compare($E a, $E b)]) {
     throw new UnsupportedError("Cannot sort immutable List.");
   }
 
   int indexOf($E element, [int start = 0]) =>
       Lists.indexOf(this, element, start, this.length);
 
@@ skipping 61 matching lines @@
   }
 
   void insertRange(int start, int rangeLength, [$E initialValue]) {
     throw new UnsupportedError("Cannot insertRange on immutable List.");
   }
 
   List<$E> getRange(int start, int rangeLength) =>
       Lists.getRange(this, start, rangeLength, <$E>[]);
 
   // -- end List<$E> mixins.