Revert "Remove Expect from core library."

tests/language/identical_closure_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() {
   // int.
   Expect.isTrue(myIdentical(42, 42));
@@ skipping 18 matching lines @@
   // Strings.
   var a = "hello";
   var b = "hello";
   // Identical strings are coalesced into single instances.
   Expect.isTrue(myIdentical(a, b));
 
   // Null handling.
   Expect.isFalse(myIdentical(42, null));
   Expect.isTrue(myIdentical(null, null));
 }