Revert "Remove Expect from core library."

tests/language/multi_assign_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.
 // Dart test program testing multiple assignment.
 
-import "package:expect/expect.dart";
-
 
 class MultiAssignTest {
   static testMain() {
     var i, j, k;
     i = j = k = 11;
     Expect.equals(11, i);
     Expect.equals(11, j);
     Expect.equals(11, k);
 
     var m;
     var n = m = k = 55;
     Expect.equals(55, m);
     Expect.equals(55, n);
     Expect.equals(55, k);
   }
 }
 
 main() {
   MultiAssignTest.testMain();
 }