Fix the build with WEB_AUDIO disabled.

tools/dom/scripts/systemnative.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 systems to generate
 native binding from the IDL database."""
 
 import emitter
 import os
 from generator import *
 from htmldartgenerator import *
-from systemhtml import js_support_checks, GetCallbackInfo
+from systemhtml import js_support_checks, GetCallbackInfo, HTML_LIBRARY_NAMES
 
 class DartiumBackend(HtmlDartGenerator):
   """Generates Dart implementation for one DOM IDL interface."""
 
   def __init__(self, interface, cpp_library_emitter, options):
     super(DartiumBackend, self).__init__(interface, options)
 
     self._interface = interface
     self._cpp_library_emitter = cpp_library_emitter
     self._database = options.database
@@ skipping 847 matching lines @@
     return self._type_registry.TypeInfo(type_name)
 
   def _DartType(self, type_name):
     return self._type_registry.DartType(type_name)
 
 
 class CPPLibraryEmitter():
   def __init__(self, emitters, cpp_sources_dir):
     self._emitters = emitters
     self._cpp_sources_dir = cpp_sources_dir
-    self._library_headers = {}
+    self._library_headers = dict((lib, []) for lib in HTML_LIBRARY_NAMES)
     self._sources_list = []
 
   def CreateHeaderEmitter(self, interface_name, library_name, is_callback=False):
     path = os.path.join(self._cpp_sources_dir, 'Dart%s.h' % interface_name)
     if not is_callback:
-      if not library_name in self._library_headers:
-        self._library_headers[library_name] = []
       self._library_headers[library_name].append(path)
     return self._emitters.FileEmitter(path)
 
   def CreateSourceEmitter(self, interface_name):
     path = os.path.join(self._cpp_sources_dir, 'Dart%s.cpp' % interface_name)
     self._sources_list.append(path)
     return self._emitters.FileEmitter(path)
 
   def EmitDerivedSources(self, template, output_dir):
     partitions = 20 # FIXME: this should be configurable.
@@ skipping 20 matching lines @@
             '    if (Dart_NativeFunction func = $CLASS_NAME::resolver(name, argumentCount))\n'
             '        return func;\n',
             CLASS_NAME=os.path.splitext(os.path.basename(path))[0])
 
 def _IsOptionalStringArgumentInInitEventMethod(interface, operation, argument):
   return (
       interface.id.endswith('Event') and
       operation.id.startswith('init') and
       argument.ext_attrs.get('Optional') == 'DefaultIsUndefined' and
       argument.type.id == 'DOMString')