- Update tests to make sure they recerefence invalid code.

dart/tests/language/src/Prefix15NegativeTest.dart

 // Copyright (c) 2011, 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.
 //
 // Unresolved symbols should be reported as an error.
 
 #library("Prefix15NegativeTest.dart");
 #import("library12.dart", prefix:"lib12");
 
 class myClass implements lib12.Library13 {
   myClass(int this.fld) : super(0);
   int fld;
 }
 
 main() {
+  new myClass(1);
 }