Update supported browser annotations for Firefox Web Audio.

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 8cdf0ea22c33601bf5821770c974c44d329de319..490140302da763d3918bb38fd2ed46103951bb45 100644
--- a/sdk/lib/web_audio/dartium/web_audio_dartium.dart
+++ b/sdk/lib/web_audio/dartium/web_audio_dartium.dart
@@ -165,8 +165,10 @@ typedef void AudioBufferCallback(AudioBuffer audioBuffer);
 
 @DocsEditable()
 @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 {
   // To suppress missing implicit constructor warnings.
   factory AudioBufferSourceNode._() { throw new UnsupportedError("Not supported"); }
@@ -263,8 +265,10 @@ class AudioBufferSourceNode extends AudioSourceNode {
 
 
 @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 {
   // To suppress missing implicit constructor warnings.
   factory AudioContext._() { throw new UnsupportedError("Not supported"); }