constructor() {
    this.reachabilityEvents = {};
    this.isClosed = () => false;
    this.isActive = () => true;
    this.shouldUseSecondaryDomains = () => false;
    this.completedRequests = [];
    this.notifyServerReachabilityEvent = function(reachabilityType) {
      if (!this.reachabilityEvents[reachabilityType]) {
        this.reachabilityEvents[reachabilityType] = 0;
      }
      this.reachabilityEvents[reachabilityType]++;
    };
    this.onRequestComplete = function(request) {
      this.completedRequests.push(request);
    };
    this.onRequestData = (request, data) => {};
  }
Constructs a duck-type BrowserChannel that tracks the completed requests.