Revert "Remove Expect from core library."

tests/language/f_bounded_quantification_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.
 
-import "package:expect/expect.dart";
-
 // Test for F-Bounded Quantification.
 
 class FBound<F extends FBound<F>> {}
 
 class Bar extends FBound<Bar> {}
 
 class SubBar extends Bar {}
 
 class Baz<T> extends FBound<Baz<T>> {}
 
@@ skipping 27 matching lines @@
     bool got_type_error = false;
     try {
       FBound<SubBaz<Bar>> fsb = new FBound<SubBaz<Bar>>();  /// 02: static type warning
     } on TypeError catch (error) {
       got_type_error = true;
     }
     // Type error in checked mode only.
     Expect.isTrue(got_type_error == isCheckedMode());  /// 02: continued
   }
 }