Revert "Remove Expect from core library."

tests/language/prefix10_negative_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.
 //
 
 // Type parameters can shadow a library prefix.
 
 library Prefix10NegativeTest.dart;
-import "package:expect/expect.dart";
 import "library10.dart" as T;
 
 class P<T> {
   P.named(T this.fld);
   T fld;
   main() {
     var i = new T.Library10(10);  // This should be an error.
     Expect.equals(10, i.fld);
   }
 }
 
 main() {
   var i = new P<int>.named(10);
   i.main();
 }