Add 'Server' and 'User-Agent' default header fields.

sdk/lib/_internal/compiler/implementation/lib/io_patch.dart

 // Copyright (c) 2013, 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.
 
 patch class _BufferUtils {
   patch static bool _isBuiltinList(List buffer) {
     throw new UnsupportedError("_isBuiltinList");
   }
 }
 
@@ skipping 285 matching lines @@
 }
 
 patch class _Filter {
   patch static _Filter newZLibDeflateFilter(bool gzip, int level) {
     throw new UnsupportedError("newZLibDeflateFilter");
   }
   patch static _Filter newZLibInflateFilter() {
     throw new UnsupportedError("newZLibInflateFilter");
   }
 }
+
+patch class _OptionsImpl {
+  patch String get version {
+    throw new UnsupportedError("_OptionsImpl.version");
+  }
+}