Make Try Dart tests run on Firefox.

dart/site/try/src/editor.dart

Index: dart/site/try/src/editor.dart
diff --git a/dart/site/try/src/editor.dart b/dart/site/try/src/editor.dart
index d330a7b7446577641d0b633b3c5b743114753837..9072ef8cc561ac74b9d9ea7da32125f8110d74fc 100644
--- a/dart/site/try/src/editor.dart
+++ b/dart/site/try/src/editor.dart
@@ -27,6 +27,9 @@ import 'decoration.dart' show
     info,
     warning;
 
+import 'selection.dart' show
+    isCollapsed;
+
 const String INDENT = '\u{a0}\u{a0}';
 
 Set<String> seenIdentifiers;
@@ -106,7 +109,7 @@ num minSuggestionWidth = 0;
 /// exists.
 Element getElementAtSelection() {
   Selection selection = window.getSelection();
-  if (!selection.isCollapsed) return null;
+  if (!isCollapsed(selection)) return null;
   var anchorNode = selection.anchorNode;
   if (!mainEditorPane.contains(anchorNode)) return null;
   if (mainEditorPane == anchorNode) return null;