Use JavaScript to detect if Selection.modify is available.

dart/site/try/src/settings.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.settings;
 
 import 'user_option.dart';
 
 const BooleanUserOption _alwaysRunInWorker =
     const BooleanUserOption('alwaysRunInWorker');
@@ skipping 83 matching lines @@
     const BooleanUserOption('incrementalCompilation');
 
 const BooleanUserOption live = const BooleanUserOption('live', isHidden: true);
 
 const BooleanUserOption alwaysRunInIframe =
     const BooleanUserOption('alwaysRunInIframe', isHidden: true);
 
 const BooleanUserOption communicateViaBlobs =
     const BooleanUserOption('communicateViaBlobs', isHidden: true);
 
+const BooleanUserOption hasSelectionModify =
+    const BooleanUserOption('hasSelectionModify', isHidden: true);
+
 const List<UserOption> options = const <UserOption>[
     _alwaysRunInWorker,
     _verboseCompiler,
     _minified,
     _onlyAnalyze,
     _enableDartMind,
     _compilationPaused,
     incrementalCompilation,
     live,
     enableCodeCompletion,
     _codeFont,
     _theme,
     _currentSample,
     alwaysRunInIframe,
     communicateViaBlobs,
+    hasSelectionModify,
   ];