CSSStyleDeclaration.getPropertyValue returns empty string instead of null.

lib/html/scripts/systemhtml.py

 #!/usr/bin/python
 # 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.
 
 """This module provides shared functionality for the system to generate
 Dart:html APIs from the IDL database."""
 
 import emitter
 
 from systembase import *
 
 _js_custom_members = set([
+    'CSSStyleDeclaration.getPropertyValue',
     'CSSStyleDeclaration.setProperty',
     'Element.insertAdjacentElement',
     'Element.insertAdjacentHTML',
     'Element.insertAdjacentText',
     'Element.remove',
     'ElementEvents.mouseWheel',
     'IDBDatabase.transaction',
     'IFrameElement.contentWindow',
     'MouseEvent.offsetX',
     'MouseEvent.offsetY',
@@ skipping 999 matching lines @@
 
     library_emitter = self._multiemitter.FileEmitter(library_file_path)
     library_file_dir = os.path.dirname(library_file_path)
     auxiliary_dir = os.path.relpath(auxiliary_dir, library_file_dir)
     imports_emitter = library_emitter.Emit(
         self._template, AUXILIARY_DIR=massage_path(auxiliary_dir))
     for path in sorted(self._path_to_emitter.keys()):
       relpath = os.path.relpath(path, library_file_dir)
       imports_emitter.Emit(
           "#source('$PATH');\n", PATH=massage_path(relpath))