WebAudio docs out of date

[efortuna]

This bug report was basically copied wholesale from an email thread:

In a conversation related to http://crbug.com/372681 it was pointed
out by the developer that Dart's docs still describe some of the
deprecated Web Audio APIs:

https://api.dartlang.org/apidocs/channels/stable/dartdoc-viewer/dart-dom-web_audio.AudioContext

Could you update the docs to describe only the current, unprefixed and non-deprecated APIs?

The WebAudio IDL was updated a little while ago so some of the functions are now gone. In particular:

createBufferFromBuffer
createDelayNode
createGainNode
createJavaScriptNode

activeSourceCount

[kevmoo]

Removed Type-Defect label.
Added Type-Documentation label.