Make Try Dart tests run on Firefox.

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)
-      : this.anchorNode = selection.isCollapsed ? selection.anchorNode : null,
-        this.anchorOffset = selection.isCollapsed ? selection.anchorOffset : -1;
+      : anchorNode = isCollapsed(selection) ? selection.anchorNode : null,
+        anchorOffset = isCollapsed(selection) ? selection.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) {
@@ skipping 29 matching lines @@
       CharacterData text = node;
       if (anchorNode == text) {
         return anchorOffset + offset;
       }
       offset += text.data.length;
     }
 
     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;
+}