Adding tests for experimental Safe DOM creation technique.

tools/dom/templates/html/impl/impl_Range.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.
 
 // WARNING: Do not edit - generated code.
 
 part of $LIBRARYNAME;
 
 $(ANNOTATIONS)class $CLASSNAME$EXTENDS$IMPLEMENTS$NATIVESPEC {
   factory $CLASSNAME() => document.$dom_createRange();
 
 $!MEMBERS
+
+  /**
+   * Checks if createContextualFragment is supported.
+   *
+   * See also:
+   *
+   * * [createContextualFragment]
+   */
+$if DART2JS
+  static bool get supportsCreateContextualFragment =>
+      JS('bool', '("createContextualFragment" in window.Range.prototype)');
+$else
+  static bool get supportsCreateContextualFragment => true;
+$endif
 }