Add missing copyright on test cases.

pkg/front_end/testcases/prefer_baseclass.dart

+// Copyright (c) 2017, 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.
+
 class A {}
 
 class B {}
 
 class AB1 extends A implements B {}
 
 class AB2 extends A implements B {}
 
 class BA1 extends B implements A {}
 
 class BA2 extends B implements A {}
 
 takeSubclassOfA(obj) {
   // The analysis should at least infer that 'obj' is a subclass of A,
   // When the upper bound is ambiguous, it should use the common superclass, if
   // there is one besides Object.
 }
 
 takeSubclassOfB(obj) {
   // Likewise, the analysis should infer that 'obj' is a subclass of B.
 }
 
 main() {
   takeSubclassOfA(new AB1());
   takeSubclassOfA(new AB2());
 
   takeSubclassOfB(new BA1());
   takeSubclassOfB(new BA2());
 }