Remove Expect from core library.

tests/compiler/dart2js_native/native_checked_fields_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 type checks occur on assignment to fields of native methods.
 
 class A native "*A" {
   int foo;
 }
 
 class B native "*B" {
   String foo;
 }
 
@@ skipping 90 matching lines @@
 
 main() {
   setup();
 
   if (isCheckedMode()) {
     checkedModeTest();
   } else {
     uncheckedModeTest();
   }
 }