Revert "Remove Expect from core library."

tests/language/generic_instanceof_test.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.
 // Test that instanceof works correctly with type variables.
 
 library GenericInstanceofTest.dart;
-import "package:expect/expect.dart";
 part "generic_instanceof.dart";
 
 main() {
   // Repeat type checks so that inlined tests can be tested as well.
   for (int i = 0; i < 5; i++) {
     GenericInstanceof.testMain();
   }
 }