constructor() {
    this.isClosed = () => false;
    this.isActive = () => true;
    this.usesFetchStreams = () => false;
    this.shouldUseSecondaryDomains = () => false;
    this.completedRequests = [];
    this.onRequestComplete = function(request) {
      this.completedRequests.push(request);
    };
    this.onRequestData = (request, data) => {};
  }
Constructs a duck-type WebChannelBase that tracks the completed requests.
@final