Fix source_update_test on Chrome.

dart/site/try/src/selection.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.selection;
 
 import 'dart:html' show
     CharacterData,
     Node,
     NodeFilter,
     Selection,
     Text,
     TreeWalker;
 
 import 'decoration.dart';
 
 class TrySelection {
   final Node root;
   Node anchorNode;
   int anchorOffset;
 
   String text;
   int globalOffset = -1;
 
   TrySelection(this.root, Selection selection)
       : anchorNode = isCollapsed(selection) ? selection.anchorNode : null,
         anchorOffset = isCollapsed(selection) ? selection.anchorOffset : -1;
 
+  TrySelection.empty(this.root)
+      : anchorNode = null,
+        anchorOffset = -1;
+
   Text addNodeFromSubstring(int start,
                             int end,
                             List<Node> nodes,
                             [Decoration decoration]) {
     if (start == end) return null;
 
     Text textNode = new Text(text.substring(start, end));
 
     if (start <= globalOffset && globalOffset <= end) {
       anchorNode = textNode;
@@ skipping 34 matching lines @@
 
     return -1;
   }
 }
 
 bool isCollapsed(Selection selection) {
   // Firefox and Chrome don't agree on if the selection is collapsed if there
   // is no node selected.
   return selection.isCollapsed && selection.anchorNode != null;
 }