Remove Expect from core library.

tests/compiler/dart2js_foreign/native_null_closure_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";
 import 'native_metadata.dart';
 
 typedef void MyFunctionType();
 
 @Native("*A")
 class A {
   @native setClosure(MyFunctionType f);
   @native check(MyFunctionType f);
   @native invoke();
 }
@@ skipping 16 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);
 }