Revert "Remove Expect from core library."

tests/language/identical_closure2_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.
 
-import "package:expect/expect.dart";
-
 var myIdentical = identical;
 
 class Point {
   num x, y;
   Point(this.x, this.y);
 }
 
 main() {
   // Bigint (2^76).
   Expect.isTrue(myIdentical(75557863725914323419136,
                             75557863725914323419136));
   Expect.isFalse(myIdentical(75557863725914323419136,
                              75557863725914323419137));
 
   // Different types.
   Expect.isFalse(myIdentical(42, 42.0));
 
   // NaN handling.
   Expect.isTrue(myIdentical(double.NAN, double.NAN));
 }