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

tools/dom/templates/html/impl/impl_Element.darttemplate

Index: tools/dom/templates/html/impl/impl_Element.darttemplate
diff --git a/tools/dom/templates/html/impl/impl_Element.darttemplate b/tools/dom/templates/html/impl/impl_Element.darttemplate
index 47f53c1499bb9d69bbeef8a90b6b01a4730e170c..468f27b3f6bbc82ceac0ee34de878f2104529e8a 100644
--- a/tools/dom/templates/html/impl/impl_Element.darttemplate
+++ b/tools/dom/templates/html/impl/impl_Element.darttemplate
@@ -145,7 +145,7 @@ class _ChildrenElementList implements List {
     }
   }
 
-  List<Element> get reversed {
+  Iterable<Element> get reversed {
     return IterableMixinWorkaround.reversedList(this);
   }
 
@@ -379,7 +379,7 @@ class _FrozenElementList implements List {
     throw new UnsupportedError('');
   }
 
-  List<Element> get reversed {
+  Iterable<Element> get reversed {
     return IterableMixinWorkaround.reversedList(this);
   }