Remove Expect from core library.

tests/compiler/dart2js_foreign/native_class_inheritance1_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.
 
 // Test to see if resolving a hidden native class's method interferes with
 // subsequent resolving the subclass's method.  This might happen if the
 // superclass caches the method in the prototype, so shadowing the dispatcher
 // stored on Object.prototype.
 
+import "package:expect/expect.dart";
 import 'native_metadata.dart';
 
 // Version 1: It might be possible to call foo directly.
 @Native("*A1")
 class A1 {
   @native foo();
 }
 
 @Native("*B1")
 class B1 extends A1  {
@@ skipping 79 matching lines @@
   caught = false;
   try {
     var x = 123;
     x.foo(20);
   } catch (ex) {
     caught = true;
     Expect.isTrue(ex is NoSuchMethodError);
   }
   Expect.isTrue(caught, "x.foo(20) should throw");
 }