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

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 implements List {
   // Raw Element.
@@ skipping 127 matching lines @@
   Element addLast(Element value) => add(value);
 
   Iterator<Element> get iterator => toList().iterator;
 
   void addAll(Iterable<Element> iterable) {
     for (Element element in iterable) {
       _element.$dom_appendChild(element);
     }
   }
 
-  List<Element> get reversed {
+  Iterable<Element> get reversed {
     return IterableMixinWorkaround.reversedList(this);
   }
 
   void sort([int compare(Element a, Element b)]) {
     throw new UnsupportedError('TODO(jacobr): should we impl?');
   }
 
   dynamic reduce(dynamic initialValue,
       dynamic combine(dynamic previousValue, Element element)) {
     return IterableMixinWorkaround.reduce(this, initialValue, combine);
@@ skipping 213 matching lines @@
   void addLast(Element value) {
     throw new UnsupportedError('');
   }
 
   Iterator<Element> get iterator => new _FrozenElementListIterator(this);
 
   void addAll(Iterable<Element> iterable) {
     throw new UnsupportedError('');
   }
 
-  List<Element> get reversed {
+  Iterable<Element> get reversed {
     return IterableMixinWorkaround.reversedList(this);
   }
 
   void sort([int compare(Element a, Element b)]) {
     throw new UnsupportedError('');
   }
 
   dynamic reduce(dynamic initialValue,
       dynamic combine(dynamic previousValue, Element element)) {
     return IterableMixinWorkaround.reduce(this, initialValue, combine);
@@ skipping 664 matching lines @@
   // Optimization to improve performance until the dart2js compiler inlines this
   // method.
   static dynamic createElement_tag(String tag) =>
       // Firefox may return a JS function for some types (Embed, Object).
       JS('Element|=Object', 'document.createElement(#)', tag);
 $else
   static Element createElement_tag(String tag) =>
       document.$dom_createElement(tag);
 $endif
 }