Revert "Remove Expect from core library."

tests/language/super_operator_index2_test.dart

 // Copyright (c) 2013, 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.
 
 // Test for super indexing operations.
 
-import "package:expect/expect.dart";
-
 class A {
   var map = new Map();
   operator[]=(a, b) { map[a] = b; }
   operator[](a) => map[a];
 }
 
 class B extends A {
   foo() {
     super[4] = 42;
     Expect.equals(42, super[4]);
     super[4] += 5;
     Expect.equals(47, super[4]);
   }
 }
 
 main() {
   new B().foo();
 }