Revert "Remove Expect from core library."

tests/language/index_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 index operators.
 
-import "package:expect/expect.dart";
-
 class Helper {
   static int fibonacci(int n) {
     int a = 0, b = 1, i = 0;
     while (i++ < n) {
       a = a + b;
       b = a - b;
     }
     return a;
   }
 }
@@ skipping 11 matching lines @@
     for (int i = 2; i < a.length; i++) {
       Expect.equals(a[i-2] + a[i-1], a[i]);
     }
     Expect.equals(515, a[3] = 515);
   }
 }
 
 main() {
   IndexTest.testMain();
 }