Use JavaScript to detect if Selection.modify is available.

dart/site/try/src/messages.dart

 // Copyright (c) 2014, 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.
 
 library trydart.messages;
 
 const Map<String, dynamic> messages = const <String, dynamic> {
   'alwaysRunInWorker':
     'Always run in Worker thread.',
 
@@ skipping 22 matching lines @@
       'Code font:',
       'Enter a size and font, for example, 11pt monospace'],
 
   'currentSample':
     'N/A',
 
   'theme':
     'Theme:',
 
   'incrementalCompilation':
-    'Enable incremental compilation (EXPERIMENTAL)',
+    'Enable incremental compilation (EXPERIMENTAL).',
+
+  'hasSelectionModify':
+    'Use Selection.modify.',
 };