Revert "Remove Expect from core library."

tests/language/is_operator_test.dart

 // Copyright (c) 2012, 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.
 // Dart test program for the "is" type test operator.
 
-import "package:expect/expect.dart";
-
 abstract class I { }
 
 abstract class AI implements I { }
 
 class A implements AI {
   const A();
 }
 
 class B implements I {
   const B();
@@ skipping 58 matching lines @@
     Expect.equals(true, null is !B);
     Expect.equals(true, null is !C);
     Expect.equals(true, null is !AI);
     Expect.equals(true, null is !I);
   }
 }
 
 main() {
   IsOperatorTest.testMain();
 }