Fix the build with WEB_AUDIO disabled.

tools/dom/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
 import monitored
 import os
 from generator import *
 from htmldartgenerator import *
 
+HTML_LIBRARY_NAMES = ['chrome', 'html', 'indexed_db', 'svg', 'web_audio',
+                      'web_gl', 'web_sql']
+
 _js_custom_members = monitored.Set('systemhtml._js_custom_members', [
     'ArrayBuffer.slice',
     'AudioBufferSourceNode.start',
     'AudioBufferSourceNode.stop',
     'AudioContext.createGain',
     'AudioContext.createScriptProcessor',
     'CanvasRenderingContext2D.drawImage',
     'CanvasRenderingContext2D.lineDashOffset',
     'Console.memory',
     'Console.profiles',
@@ skipping 1118 matching lines @@
     for library_name in libraries:
       self._libraries[library_name] = DartLibrary(
           library_name, template_loader, library_type, output_dir)
 
   def AddFile(self, basename, library_name, path):
     self._libraries[library_name].AddFile(path)
 
   def Emit(self, emitter, auxiliary_dir):
     for lib in self._libraries.values():
       lib.Emit(emitter, auxiliary_dir)