Make new StreamController be async by default.

sdk/lib/web_audio/dartium/web_audio_dartium.dart

Index: sdk/lib/web_audio/dartium/web_audio_dartium.dart
diff --git a/sdk/lib/web_audio/dartium/web_audio_dartium.dart b/sdk/lib/web_audio/dartium/web_audio_dartium.dart
index 786b08dd65fc6efdee8de584f4078f042fa68ecf..2eefb8c1551c62dabf736ba9789cecd23267013c 100644
--- a/sdk/lib/web_audio/dartium/web_audio_dartium.dart
+++ b/sdk/lib/web_audio/dartium/web_audio_dartium.dart
@@ -1258,8 +1258,8 @@ class ScriptProcessorNode extends AudioNode {
    */
   Stream<AudioProcessingEvent> get onAudioProcess {
     if (_eventStream == null) {
-      var controller = new StreamController();
-      var callback = (audioData) { 
+      var controller = new StreamController(sync: true);
+      var callback = (audioData) {
           if (controller.hasListener) {
             // This stream is a strange combination of broadcast and single
             // subscriber streams. We only allow one listener, but if there is