docs: function(nextComponent, container) {
      ("production" !== process.env.NODE_ENV ? invariant(
        container && (
          (container.nodeType === ELEMENT_NODE_TYPE || container.nodeType === DOC_NODE_TYPE)
        ),
        '_registerComponent(...): Target container is not a DOM element.'
      ) : invariant(container && (
        (container.nodeType === ELEMENT_NODE_TYPE || container.nodeType === DOC_NODE_TYPE)
      )));
  
      ReactBrowserEventEmitter.ensureScrollValueMonitoring();
  
      var reactRootID = ReactMount.registerContainer(container);
      instancesByReactRootID[reactRootID] = nextComponent;
      return reactRootID;
    }