Revert "Remove Expect from core library."

tests/language/bad_initializer2_negative_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.
 // Variable initializer must not reference the initialized variable.
 
-import "package:expect/expect.dart";
-
 class BadInitializer2NegativeTest {
   static testMain() {
    var foo = (int n) {
      if (n == 0) {
        return 0;
      } else {
        return 1 + foo(n - 1);  // <-- self-reference to closure foo.
      }
    };
    Expect.equals(4, foo(4));
   }
 }
 
 main() {
   BadInitializer2NegativeTest.testMain();
 }