Revert "Remove Expect from core library."

tests/language/closure_parameter_types_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";
-
 // Regression test for dart2js, where the optimizer was too agressive
 // about parameter types of closures.
 
 class A {
   Function f;
   A(this.f);
   _do() => f(1);
 }
 
 main() {
   int invokeCount = 0;
   closure(a) {
     if (invokeCount++ == 1) {
       Expect.isTrue(a is int);
     }
   }
   closure('s');
   new A(closure)._do();
   Expect.equals(2, invokeCount);
 }