Revert "Remove Expect from core library."

tests/compiler/dart2js_native/native_null_closure_frog_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";
-
 typedef void MyFunctionType();
 
 class A native "*A" {
   setClosure(MyFunctionType f) native;
   check(MyFunctionType f) native;
   invoke() native;
 }
 
 makeA() native { return new A(); }
 
@@ skipping 12 matching lines @@
   a.setClosure(null);
   Expect.isTrue(a.check(null));
   bool caughtException = false;
   try {
     a.invoke();
   } on Exception catch (e) {
     caughtException = true;
   }
   Expect.isTrue(caughtException);
 }