Fix Win64 build of Dart VM.

runtime/bin/net/os_windows.c

Index: runtime/bin/net/os_windows.c
diff --git a/pkg/docgen/lib/src/exports/mirrors_util.dart b/runtime/bin/net/os_windows.c
similarity index 54%
copy from pkg/docgen/lib/src/exports/mirrors_util.dart
copy to runtime/bin/net/os_windows.c
index 2479fbe304ee551b1ef3d023356bd5d1319f96a4..07ccdc50c06c7c9339688ccf54cf5e5b6bba1721 100644
--- a/pkg/docgen/lib/src/exports/mirrors_util.dart
+++ b/runtime/bin/net/os_windows.c
@@ -2,6 +2,10 @@
 // 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.
 
-library docgen.exports.mirrors_util;
+#if !defined(_MSC_VER)
+#error "This file should only be compiled under MSVC"
+#endif
 
-export '../../../../../sdk/lib/_internal/compiler/implementation/mirrors/mirrors_util.dart';
+#if defined(_X86_)
+#include "../../../third_party/nss/nss/lib/freebl/mpi/mpi_x86_asm.c"
+#endif