code: function Registry(ctx, prefixLength) {
    this._ctx = ctx; // Context for all callbacks.
    this._mask = ~0 >>> (prefixLength | 0); // 16 bits by default.
    this._id = 0; // Unique integer ID for each call.
    this._n = 0; // Number of pending calls.
    this._cbs = {};
  }
docs: Callback registry.
  
  Callbacks added must accept an error as first argument. This is used by
  client channels to store pending calls. This class isn't exposed by the
  public API.