Merge more Kernel infrastructure from kernel_sdk SDK fork.

runtime/bin/nativewrappers.dart

+// Copyright (c) 2016, 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.
+
+// This library is only used by the Kernel frontend.
+// VM creates these classes in Library::InitNativeWrappersLibrary.
+
+library dart.nativewrappers;
+
+class NativeFieldWrapperClass1 extends Object {}
+class NativeFieldWrapperClass2 extends Object {}
+class NativeFieldWrapperClass3 extends Object {}
+class NativeFieldWrapperClass4 extends Object {}