Make Try Dart tests run on Firefox.

dart/site/try/src/selection.dart

Index: dart/site/try/src/selection.dart
diff --git a/dart/site/try/src/selection.dart b/dart/site/try/src/selection.dart
index 35e17a8bab13d09753ca7ad3746f5b0f3a7fc9a7..bf48c6d217b9095cf33852c62cce6a2c7707c6c4 100644
--- a/dart/site/try/src/selection.dart
+++ b/dart/site/try/src/selection.dart
@@ -23,8 +23,8 @@ class TrySelection {
   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,
@@ -74,3 +74,9 @@ class TrySelection {
     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;
+}