Revert "Remove Expect from core library."

tests/language/list_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 for testing arrays.
 
-import "package:expect/expect.dart";
-
 class ListTest {
   static void TestIterator() {
     List<int> a = new List<int>(10);
     int count = 0;
 
     // Basic iteration over ObjectList.
     for (int elem in a) {
       Expect.equals(null, elem);
       count++;
     }
@@ skipping 91 matching lines @@
     // We cannot write just 'list.removeLast' due to issue 3769.
     Expect.throws(() => list.removeLast(),
                   (e) => e is RangeError);
     Expect.equals(0, list.length);
   }
 }
 
 main() {
   ListTest.testMain();
 }