Implement getRange (returning an Iterable).

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 165 matching lines @@
   }
 
   void removeRange(int start, int rangeLength) {
     throw new UnsupportedError("Cannot removeRange on immutable List.");
   }
 
   void insertRange(int start, int rangeLength, [$E initialValue]) {
     throw new UnsupportedError("Cannot insertRange on immutable List.");
   }
 
+  Iterable<$E> getRange(int start, int end) =>
+    IterableMixinWorkaround.getRangeList(this, start, end);
+
   List<$E> sublist(int start, [int end]) {
     if (end == null) end = length;
     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);
 
   String toString() {
     StringBuffer buffer = new StringBuffer('[');
     buffer.writeAll(this, ', ');
     buffer.write(']');
     return buffer.toString();
   }
 
   // -- end List<$E> mixins.