Turn on webaudio for JS backends.

client/dom/generated/src/interface/AudioGain.dart

Index: client/dom/generated/src/interface/AudioGain.dart
diff --git a/client/dom/generated/src/interface/AudioGain.dart b/client/dom/generated/src/interface/AudioGain.dart
new file mode 100644
index 0000000000000000000000000000000000000000..b4d773fc60cb4719f386a3ff127fcdf7aaac1265
--- /dev/null
+++ b/client/dom/generated/src/interface/AudioGain.dart
@@ -0,0 +1,8 @@
+// Copyright (c) 2011, 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.
+
+// WARNING: Do not edit - generated code.
+
+interface AudioGain extends AudioParam {
+}