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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
global: 'ReactTestRenderer',
    externals: ['react'],
  },
  {
    label: 'test-shallow',
    bundleTypes: [FB_DEV, NODE_DEV, NODE_PROD],
    moduleType: RENDERER,
    entry: 'react-test-renderer/shallow',
    global: 'ReactShallowRenderer',
    externals: ['react'],
  },
  
  {
    label: 'noop',
    bundleTypes: [NODE_DEV, NODE_PROD],
    moduleType: RENDERER,
    entry: 'react-noop-renderer',
    global: 'ReactNoopRenderer',
    externals: ['react', 'expect'],
    
    
    
    
    
    
    
    babel: opts =>
      Object.assign({}, opts, {
        plugins: opts.plugins.concat([
          require.resolve('babel-plugin-transform-regenerator'),