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**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
'use strict';
const bundleTypes = {
  UMD_DEV: 'UMD_DEV',
  UMD_PROD: 'UMD_PROD',
  NODE_DEV: 'NODE_DEV',
  NODE_PROD: 'NODE_PROD',
  FB_DEV: 'FB_DEV',
  FB_PROD: 'FB_PROD',
  RN_DEV: 'RN_DEV',
  RN_PROD: 'RN_PROD',
};
const UMD_DEV = bundleTypes.UMD_DEV;
const UMD_PROD = bundleTypes.UMD_PROD;
const NODE_DEV = bundleTypes.NODE_DEV;
const NODE_PROD = bundleTypes.NODE_PROD;
const FB_DEV = bundleTypes.FB_DEV;
const FB_PROD = bundleTypes.FB_PROD;
const RN_DEV = bundleTypes.RN_DEV;
const RN_PROD = bundleTypes.RN_PROD;
const moduleTypes = {
  ISOMORPHIC: 'ISOMORPHIC',
  RENDERER: 'RENDERER',
  RENDERER_UTILS: 'RENDERER_UTILS',