Add native extensions to the Dart shell, on the linux platform.

runtime/bin/extensions_macos.cc

Index: runtime/bin/extensions_macos.cc
diff --git a/runtime/bin/extensions_macos.cc b/runtime/bin/extensions_macos.cc
new file mode 100644
index 0000000000000000000000000000000000000000..d81e670eef10218fe7bc68228faa4d0d88a9083c
--- /dev/null
+++ b/runtime/bin/extensions_macos.cc
@@ -0,0 +1,14 @@
+// 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.
+
+#include "bin/extensions.h"
+
+#include "include/dart_api.h"
+#include "platform/assert.h"
+
+Dart_Handle Extensions::LoadExtension(const char* extension_url,
+                                      Dart_Handle parent_library) {
+  UNIMPLEMENTED();
+  return NULL;
+}