Revert "Remove Expect from core library."

tests/corelib/collection_contains_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";
-
 // Tests for the contains methods on lists.
 
 test(list, notInList) {
   testList(list) {
     for (int i = 0; i < list.length; i++) {
       var elem = list[i];
       Expect.isTrue(list.contains(list[i]), "$list.contains($elem)");
     }
     Expect.isFalse(list.contains(notInList), "!$list.contains($notInList)");
   }
@@ skipping 21 matching lines @@
   test(const <int>[1, 2, 3, null], 0);
   test(const <bool>[true, false], null);
   test(const <C>[const C(), const C(), null], new C());
   test(<C>[new C(), new C(), new C(), null], new C());
   test(const <double>[ 0.0, 1.0, 5e-324, 1e+308, double.INFINITY ], 2.0);
   Expect.isTrue(const <double>[-0.0].contains(0.0));
   Expect.isFalse(const <double>[double.NAN].contains(double.NAN));
   var niet = new Niet();
   Expect.isFalse([niet].contains(niet));
 }