static proxyHandler_(target, methodName, receiver) {
    const handle = target[handleSymbol];
    if (methodName === handleSymbol)
      return handle;
    if (typeof methodName !== 'string')
      return;
    if (methodName === 'then')
      return target[methodName];
    return handle.callMethod_.bind(handle, methodName);
  }
We always return proxies to the user to encapsulate handle and marshall
calls automatically.