Revert "Remove Expect from core library."

tests/language/super_operator_index3_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 operator[]= resolved in super class.
 
-import "package:expect/expect.dart";
-
 class A {
   var indexField = new List(2);
   operator[]=(index, value) { indexField[index] = value; }
 }
 
 class B extends A {
   operator[](index) => indexField[index];
 }
 
 class C extends B {
@@ skipping 10 matching lines @@
     Expect.equals(3, super[0] += 1);
     Expect.equals(3, super[0]);
     Expect.equals(3, super[0]++);
     Expect.equals(4, super[0]);
   }
 }
 
 main() {
   new C().test();
 }