Add native extensions for the Dart shell to mac and windows

runtime/bin/test_extension_dllmain_win.cc

Index: runtime/bin/test_extension_dllmain_win.cc
diff --git a/runtime/bin/test_extension_dllmain_win.cc b/runtime/bin/test_extension_dllmain_win.cc
new file mode 100644
index 0000000000000000000000000000000000000000..fdbd17ed4a55206a8f074fce130cfc7629b6d111
--- /dev/null
+++ b/runtime/bin/test_extension_dllmain_win.cc
@@ -0,0 +1,12 @@
+// 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.
+
+#define WIN32_LEAN_AND_MEAN
+#include <windows.h>
+
+BOOL APIENTRY DllMain(HMODULE module,
+                      DWORD  reason,
+                      LPVOID reserved) {
+  return true;
+}