constructor(dict, keys, xref) {
    this.dict = dict;
    this.keys = keys;
    this.xref = xref;
  }
A helper for loading missing data in `Dict` graphs. It traverses the graph
depth first and queues up any objects that have missing data. Once it has
has traversed as many objects that are available it attempts to bundle the
missing data requests and then resume from the nodes that weren't ready.

NOTE: It provides protection from circular references by keeping track of
loaded references. However, you must be careful not to load any graphs
that have references to the catalog or other pages since that will cause the
entire PDF document object graph to be traversed.