<!--
  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: 'art',
    bundleTypes: [UMD_DEV, UMD_PROD, NODE_DEV, NODE_PROD, FB_DEV, FB_PROD],
    moduleType: RENDERER,
    entry: 'react-art',
    global: 'ReactART',
    externals: ['react'],
    babel: opts =>
      Object.assign({}, opts, {
        
        presets: opts.presets.concat([require.resolve('babel-preset-react')]),
      }),
  },
  
  {
    label: 'native',
    bundleTypes: [RN_DEV, RN_PROD],
    moduleType: RENDERER,
    entry: 'react-native-renderer',
    global: 'ReactNativeRenderer',
    externals: [
      'ExceptionsManager',
      'InitializeCore',
      'Platform',
      'RCTEventEmitter',
      'TextInputState',
      'UIManager',
      'View',
      'deepDiffer',
      'deepFreezeAndThrowOnMutationInDev',