Don't put private members in dart:html interfaces.

client/html/src/_Testing.dart

Index: client/html/src/_Testing.dart
diff --git a/client/html/src/_Testing.dart b/client/html/src/_Testing.dart
index 0252adaa9e5e9677ebf9b3db139f489d027b69b7..7313c033e8944408ebd4abc2b9e08a01dc152dfd 100644
--- a/client/html/src/_Testing.dart
+++ b/client/html/src/_Testing.dart
@@ -8,10 +8,12 @@
  */
 class Testing {
   static void addEventListener(EventTarget target, String type, EventListener listener, bool useCapture) {
-    target._addEventListener(type, listener, useCapture);
+    final _EventTargetImpl targetImpl = target;
+    targetImpl._addEventListener(type, listener, useCapture);
   }
   static void removeEventListener(EventTarget target, String type, EventListener listener, bool useCapture) {
-    target._removeEventListener(type, listener, useCapture);
+    final _EventTargetImpl targetImpl = target;
+    targetImpl._removeEventListener(type, listener, useCapture);
   }
 
 }