We should be able to look at see if the child has a data-react-id attribute but isn't rendered by this copy of React and consequently warn.

See https://groups.google.com/forum/#!topic/reactjs/mdwfKKJ0L2Q.

var root = getHostRootInstanceInContainer(container);
  return root ? root._hostContainerInfo._topLevelWrapper : null;
}
/**
 * Temporary (?) hack so that we can store all top-level pending updates on
 * composites instead of having to worry about different types of components
 * here.
 */
var topLevelRootCounter = 1;
var TopLevelWrapper = function() {
  this.rootID = topLevelRootCounter++;
};
TopLevelWrapper.prototype.isReactComponent = {};
if (__DEV__) {
  TopLevelWrapper.displayName = 'TopLevelWrapper';
}
TopLevelWrapper.prototype.render = function() {
  return this.props.child;
};
TopLevelWrapper.isReactTopLevelWrapper = true;
/**
 * Mounting is the process of initializing a React component by creating its
 * representative DOM elements and inserting them into a supplied `container`.
 * Any prior content inside `container` is destroyed in the process.