Revert "Remove Expect from core library."

tests/language/super_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";
-
 int i = 0;
 
 // Tests super calls and constructors.
 main() {
   Sub sub = new Sub(1, 2);
   Expect.equals(1, sub.x);
   Expect.equals(2, sub.y);
   Expect.equals(3, sub.z);
   Expect.equals(1, sub.v);
   Expect.equals(2, sub.w);
@@ skipping 24 matching lines @@
   var u, v, w;
 
   Sub(a, b) : super(a, b), this.v = a, this.w = b {
     u = a + b;
   }
 
   Sub.stat() : super.stat(), this.v = i++, this.w = i++ {
     u = i++;
   }
 }