<!--
  Note: if the issue is about documentation or the website, please file it at:
  https://github.com/reactjs/reactjs.org/issues/new
-->

**Do you want to request a *feature* or report a *bug*?**

I want to request a feature.

**What is the current behavior?**

We have no API to verify a ReactNode is a React 16 Fragment. Though now we can use `React.isValidElement(instance) && typeof instance.type === 'symbol'` to distinguish it. It's verbose and seems uncertianly right.

**What is the expected behavior?**

Add api:

```
React.isFragment(object)
```
Verifies the object is a React.Fragment. Returns true or false.

**Which versions of React, and which browser / OS are affected by this issue? Did this work in previous versions of React?**

React 16.2
{
    label: 'react-call-return',
    bundleTypes: [NODE_DEV, NODE_PROD],
    moduleType: ISOMORPHIC,
    entry: 'react-call-return',
    global: 'ReactCallReturn',
    externals: [],
  },
];
function deepFreeze(o) {
  Object.freeze(o);
  Object.getOwnPropertyNames(o).forEach(function(prop) {
    if (
      o[prop] !== null &&
      (typeof o[prop] === 'object' || typeof o[prop] === 'function') &&
      !Object.isFrozen(o[prop])
    ) {
      deepFreeze(o[prop]);
    }
  });
  return o;
}
deepFreeze(bundles);
module.exports = {
  bundleTypes,
  moduleTypes,
  bundles,
};