Remove Expect from core library.

tests/compiler/dart2js_foreign/native_class_with_dart_methods_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.
 
 // Additional Dart code may be 'placed on' hidden native classes.
 
+import "package:expect/expect.dart";
 import 'native_metadata.dart';
 
 @Native("*A")
 class A {
 
   var _field;
 
   int get X => _field;
   void set X(int x) { _field = x; }
 
@@ skipping 11 matching lines @@
 
 main() {
   setup();
 
   var a = makeA();
 
   a.X = 100;
   Expect.equals(100, a.X);
   Expect.equals(150, a.method(50));
 }