Update supported browser annotations for Firefox Web Audio.

sdk/lib/web_audio/dart2js/web_audio_dart2js.dart

Index: sdk/lib/web_audio/dart2js/web_audio_dart2js.dart
diff --git a/sdk/lib/web_audio/dart2js/web_audio_dart2js.dart b/sdk/lib/web_audio/dart2js/web_audio_dart2js.dart
index cc567427ae9e7051ceff7c8f34298524dbd2166c..5fdb510a4ed0a9758099444a82dd45da7a5b4ff6 100644
--- a/sdk/lib/web_audio/dart2js/web_audio_dart2js.dart
+++ b/sdk/lib/web_audio/dart2js/web_audio_dart2js.dart
@@ -115,8 +115,10 @@ typedef void AudioBufferCallback(AudioBuffer audioBuffer);
 
 
 @DomName('AudioBufferSourceNode')
-// https://dvcs.w3.org/hg/audio/raw-file/tip/webaudio/specification.html#AudioBufferSourceNode-section
+@SupportedBrowser(SupportedBrowser.CHROME)
+@SupportedBrowser(SupportedBrowser.FIREFOX)
 @Experimental()
+// https://dvcs.w3.org/hg/audio/raw-file/tip/webaudio/specification.html#AudioBufferSourceNode-section
 class AudioBufferSourceNode extends AudioSourceNode native "AudioBufferSourceNode" {
 
   // TODO(efortuna): Remove these methods when Chrome stable also uses start
@@ -223,8 +225,10 @@ class AudioBufferSourceNode extends AudioSourceNode native "AudioBufferSourceNod
 
 
 @DomName('AudioContext')
-// https://dvcs.w3.org/hg/audio/raw-file/tip/webaudio/specification.html#AudioContext-section
+@SupportedBrowser(SupportedBrowser.CHROME)
+@SupportedBrowser(SupportedBrowser.FIREFOX)
 @Experimental()
+// https://dvcs.w3.org/hg/audio/raw-file/tip/webaudio/specification.html#AudioContext-section
 class AudioContext extends EventTarget native "AudioContext,webkitAudioContext" {
   // To suppress missing implicit constructor warnings.
   factory AudioContext._() { throw new UnsupportedError("Not supported"); }