Adding 'part of html' to all files which are used by dart:html.

sdk/lib/html/templates/html/dart2js/impl_SelectElement.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 html;
+
 class $CLASSNAME$EXTENDS$IMPLEMENTS$NATIVESPEC {
 $!MEMBERS
 
   // Override default options, since IE returns SelectElement itself and it
   // does not operate as a List.
   List<OptionElement> get options {
     return this.elements.filter((e) => e is OptionElement);
   }
 
   List<OptionElement> get selectedOptions {
     // IE does not change the selected flag for single-selection items.
     if (this.multiple) {
       return this.options.filter((o) => o.selected);
     } else {
       return [this.options[this.selectedIndex]];
     }
   }
 }