Called when the user makes a call to tts.speak() and one of the voices from this extension's manifest is the first to match the options object.

class OnSpeakEvent {
 /**
  * The text to speak, specified as either plain text or an SSML document. If
  * your engine does not support SSML, you should strip out all XML markup and
  * synthesize only the underlying text content. The value of this parameter is
  * guaranteed to be no more than 32,768 characters. If this engine does not
  * support speaking that many characters at a time, the utterance should be
  * split into smaller chunks and queued internally without returning an error.
  */
 final String utterance;

 /**
  * Options specified to the tts.speak() method.
  */
 final Map options;

 /**
  * Call this function with events that occur in the process of speaking the
  * utterance.
  */
 final dynamic sendTtsEvent;

 OnSpeakEvent(this.utterance, this.options, this.sendTtsEvent);
}