An example of how you could calculate wait time without modifying or extending dash.js

dash-init-wait-time-calculation.js

(function(){
  var videoElement = document.querySelector("#videoPlayer");
  var url = "http://dash.edgesuite.net/envivio/Envivio-dash2/manifest.mpd";
  var player = MediaPlayer().create(); 
  player.initialize(videoElement, url, true);
  
  var waitTimeStart;
  videoElement.addEventListener('waiting', function() {
    // If a second stream stalls keep the oldest start time
    if (!waitTimeStart) {
      waitTimeStart = performance.now();
    }
  });
  videoElement.addEventListener('playing', function() {
    if (waitTimeStart) {
      console.log('Stream was stalled for: ', performance.now() - waitTimeStart);
      waitTimeStart = null;
    }
  });
})();