Fix the build with WEB_AUDIO disabled.

tools/dom/idl/dart/dart.idl

Index: tools/dom/idl/dart/dart.idl
diff --git a/tools/dom/idl/dart/dart.idl b/tools/dom/idl/dart/dart.idl
index 0b1745700991a4f21b75923e8b30d626086311cd..a6f2060b2805afd63f9d433fabc1dfbf534ca92c 100644
--- a/tools/dom/idl/dart/dart.idl
+++ b/tools/dom/idl/dart/dart.idl
@@ -1,7 +1,8 @@
 // This file introduces / supplements and forces Dart declarations.
 
 [Supplemental,
- Constructor]
+ Constructor,
+ Conditional=WEB_AUDIO]
 interface AudioContext {};
 
 [Supplemental]
@@ -10,7 +11,8 @@ interface Document {
   CanvasRenderingContext getCSSCanvasContext(in DOMString contextId, in DOMString name, in long width, in long height);
 };
 
-[Supplemental]
+[Supplemental,
+ Conditional=WEB_AUDIO]
 interface ScriptProcessorNode {
   [Suppressed] attribute EventListener onaudioprocess;
   [Custom] void _setEventListener(EventListener eventListener);