Remove Expect from core library.

tests/compiler/dart2js_native/native_class_avoids_hidden_name_frog_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.
 
+import "package:expect/expect.dart";
+
 // Test that hidden native class names are not used by generated code.
 
 class A native "*B" {
   get name => 'A';
   static A create() => makeA();
 }
 
 class B native "*C" {
   get name => 'B';
   static B create() => makeB();
@@ skipping 30 matching lines @@
 
   var things = [A.create(), B.create(), C.create()];
   var a = things[inscrutable(0)];
   var b = things[inscrutable(1)];
   var c = things[inscrutable(2)];
 
   Expect.equals('A', a.name);
   Expect.equals('B', b.name);
   Expect.equals('C', c.name);
 }