xmlHttpGet(uri, decodeChunks, hostPrefix) {
    this.channelDebug_.debug(
        `<--- GET: ${uri}, ${decodeChunks}, ${hostPrefix}`);
    this.requestStartTime_ = Date.now();
  }
@param {?Uri} uri The uri of the request.
@param {boolean} decodeChunks Whether to the result is expected to be
    encoded for chunking and thus requires decoding.
@param {?string} hostPrefix The host prefix, if we might be using a
    secondary domain. Note that it should also be in the URL, adding this
    won't cause it to be added to the URL.