Fix the build with WEB_AUDIO disabled.

tools/dom/idl/dart/dart.idl

 // This file introduces / supplements and forces Dart declarations.
 
 [Supplemental,
- Constructor]
+ Constructor,
+ Conditional=WEB_AUDIO]
 interface AudioContext {};
 
 [Supplemental]
 interface Document {
   [Suppressed] DOMObject getCSSCanvasContext(in DOMString contextId, in DOMString name, in long width, in long height);
   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);
 };
 
 // Force ElementTraversal. WebKit defines these directly.
 interface ElementTraversal {
   readonly attribute unsigned long childElementCount;
   readonly attribute Element firstElementChild;
   readonly attribute Element lastElementChild;
@@ skipping 325 matching lines @@
        raises(DOMException);
    [Custom] void send([StrictTypeChecking] in DOMString data)
        raises(DOMException);
    [Custom] void send(in DOMFormData data)
        raises(DOMException);
 };
 
 
 [Suppressed]
 interface Entity {};