Revert "Remove Expect from core library."

tests/compiler/dart2js_extra/optional_parameter_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";
-
 class A {
   foo([a]) {
     return a;
   }
 
   bar(a, [b]) {
     return b;
   }
 
   noSuchMethod(mirror) {
     return 0;
   }
 }
 
 main() {
   A a = new A();
   Expect.equals(42, a.foo(42));
   Expect.equals(null, a.foo());
   Expect.equals(null, a.bar(42));
   Expect.equals(42, a.bar(null, 42));
   Expect.equals(0, a.foo(1, 2));
   Expect.equals(0, a.bar());
   Expect.equals(0, a.bar(1, 2, 3));
 }