anchor.diff:  
       // Upon getting certain special marks we create some special episodes.
       if (markName == FIRST_BYTE) {
  -      measure(BACK_END, START_TIME, FIRST_BYTE);
  +      if (_marks.containsKey(START_TIME)) {
  +        measure(BACK_END, START_TIME, FIRST_BYTE);
  +      }
       } else if (markName == ON_LOAD) {
  -      measure(FRONT_END, FIRST_BYTE, ON_LOAD);
  -      measure(PAGE_LOAD_TIME, START_TIME, ON_LOAD);
  +      if (_marks.containsKey(FIRST_BYTE)) {
  +        measure(FRONT_END, FIRST_BYTE, ON_LOAD);
  +      }
  +      if (_marks.containsKey(START_TIME)) {
  +        measure(PAGE_LOAD_TIME, START_TIME, ON_LOAD);
  +      }
       } else if (markName == DONE) {
  -      measure(TOTAL_TIME, START_TIME);
  +      if (_marks.containsKey(START_TIME)) {
  +        measure(TOTAL_TIME, START_TIME);
  +      }
       }
     }
   
  
positive.code:         _includePerfMarks = includePerfMarks,
          _autorun = autorun,
          _debug = debug {
      _targetOrigin =
          '${window.location.protocol}//'
          '${window.location.hostname}:${window.location.port}';
      // If we're running locally we can get a file://-style target
      // origin which can blow up window.postMessage, so fix that.
      if (_targetOrigin.startsWith('file://')) {
        _targetOrigin = '*';
      }
      _marks = {};
      _measures = {};
      // We need to save the starts so that given a measure we can
      // say the epoch times that it began and ended.
      _starts = {};
      // Get the start time. This can come from
      // a cookie, or from window.performance.timing.
      var startTime = findStartTime();
  
negative.code:   * implementation). If this is false, then we would typically
    * use dumpEpisodes or dumpMarks to get the performance data.
    */
    final bool _postMessages;
  
   /**
    * [_autorun] controls whether or not we will automatically call
    * done() when window.onLoad fires.
    */
    final bool _autorun;
  
   /**
    * [_includePerfMarks] controls whether or not to create a set of
    * marks and intervals (episodes) from the window.performance.timing
    * statistics.
    */
    final bool _includePerfMarks;
  
   /** [_debug] controls whether or not to print debug logs.  */
    final bool _debug;
  
   /**
    * To use the library an instance of the Episodes class
  