Convert decodeAudioData to return a Future.

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 93d19303f0a8829d54e10439263b0b35859a8019..8fb7c34a4cb30a89e79a353a47e0986a7f3716d0 100644
--- a/sdk/lib/web_audio/dartium/web_audio_dartium.dart
+++ b/sdk/lib/web_audio/dartium/web_audio_dartium.dart
@@ -432,7 +432,15 @@ class AudioContext extends EventTarget {
 
   @DomName('AudioContext.decodeAudioData')
   @DocsEditable()
-  void decodeAudioData(ByteBuffer audioData, AudioBufferCallback successCallback, [AudioBufferCallback errorCallback]) native "AudioContext_decodeAudioData_Callback";
+  void _decodeAudioData(ByteBuffer audioData, AudioBufferCallback successCallback, [AudioBufferCallback errorCallback]) native "AudioContext_decodeAudioData_Callback";
+
+  Future<AudioBuffer> decodeAudioData(ByteBuffer audioData) {
+    var completer = new Completer<AudioBuffer>();
+    _decodeAudioData(audioData,
+        (value) { completer.complete(value); },
+        (error) { completer.completeError(error); });
+    return completer.future;
+  }
 
   @DomName('AudioContext.startRendering')
   @DocsEditable()