Revert "Remove Expect from core library."

tests/language/super_all_named_constructor_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 for testing implicit invocation of super constructor with all
 // named parameters.
 
-import "package:expect/expect.dart";
-
 var res = 0;
 
 class A {
   A([v = 1]) {
     res += v;
   }
 }
 
 class B extends A {
   B([v = 2]) {
     res += v;
   }
 }
 
 main() {
   new B();
   Expect.equals(3, res);
 }