Use _downcast and _cast instead of "as" to reduce dart2js output size.

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;
 
 class _ChildrenElementList extends ListBase<Element>
     implements NodeListWrapper {
   // Raw Element.
   final Element _element;
@@ skipping 262 matching lines @@
 
   _FrozenElementList._wrap(this._nodeList) {
       this.dartClass_instance = this._nodeList;
   }
 $else
   _FrozenElementList._wrap(this._nodeList);
 $endif
 
   int get length => _nodeList.length;
 
-  E operator [](int index) => _nodeList[index] as E;
+  E operator [](int index) => _downcast/*<Node, E>*/(_nodeList[index]);
 
   void operator []=(int index, E value) {
     throw new UnsupportedError('Cannot modify list');
   }
 
   set length(int newLength) {
     throw new UnsupportedError('Cannot modify list');
   }
 
   void sort([Comparator<E> compare]) {
     throw new UnsupportedError('Cannot sort list');
   }
 
   void shuffle([Random random]) {
     throw new UnsupportedError('Cannot shuffle list');
   }
 
-  E get first => _nodeList.first as E;
+  E get first => _downcast/*<Node, E>*/(_nodeList.first);
 
-  E get last => _nodeList.last as E;
+  E get last => _downcast/*<Node, E>*/(_nodeList.last);
 
-  E get single => _nodeList.single as E;
+  E get single => _downcast/*<Node, E>*/(_nodeList.single);
 
   CssClassSet get classes => new _MultiElementCssClassSet(this);
 
   CssStyleDeclarationBase get style =>
       new _CssStyleDeclarationSet(this);
 
   set classes(Iterable<String> value) {
     // TODO(sra): This might be faster for Sets:
     //
     //     new _MultiElementCssClassSet(this).writeClasses(value)
@@ skipping 1353 matching lines @@
   const ScrollAlignment._internal(this._value);
   toString() => 'ScrollAlignment.$_value';
 
   /// Attempt to align the element to the top of the scrollable area.
   static const TOP = const ScrollAlignment._internal('TOP');
   /// Attempt to center the element in the scrollable area.
   static const CENTER = const ScrollAlignment._internal('CENTER');
   /// Attempt to align the element to the bottom of the scrollable area.
   static const BOTTOM = const ScrollAlignment._internal('BOTTOM');
 }