Making Element.matches correctly fail on IE9.

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 html;
 
 // TODO(jacobr): use _Lists.dart to remove some of the duplicated
 // functionality.
 class _ChildrenElementList implements List {
   // Raw Element.
@@ skipping 900 matching lines @@
   bool matches(String selectors) {
     if (JS('bool', '!!#.matches', this)) {
       return JS('bool', '#.matches(#)', this, selectors);
     } else if (JS('bool', '!!#.webkitMatchesSelector', this)) {
       return JS('bool', '#.webkitMatchesSelector(#)', this, selectors);
     } else if (JS('bool', '!!#.mozMatchesSelector', this)) {
       return JS('bool', '#.mozMatchesSelector(#)', this, selectors);
     } else if (JS('bool', '!!#.msMatchesSelector', this)) {
       return JS('bool', '#.msMatchesSelector(#)', this, selectors);
     }
+    throw new UnsupportedError("Not supported on this platform");
   }
 
   /**
    * Checks if the matches function is supported on the current platform.
    *
    * See also:
    *
    * * [matches]
    */
   static bool get supportsMatches {
@@ skipping 145 matching lines @@
 $if DART2JS
   // Optimization to improve performance until the dart2js compiler inlines this
   // method.
   static dynamic createElement_tag(String tag) =>
       JS('Element', 'document.createElement(#)', tag);
 $else
   static Element createElement_tag(String tag) =>
       document.$dom_createElement(tag);
 $endif
 }