Remove Expect from core library.

tests/compiler/dart2js_foreign/native_missing_method2_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';
 
 @Native("*A")
 class A {
 }
 
 @native makeA();
 
 @Native("""
 function A() {};
@@ skipping 32 matching lines @@
                 (e) => e is NoSuchMethodError);
   Expect.throws(() => a.foo = 4,
                 (e) => e is NoSuchMethodError);
 }
 
 main() {
   setup();
   typedContext();
   untypedContext();
 }