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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
browser: true,
  },
  globals: {
    
    Map: true,
    Set: true,
    Symbol: true,
    Proxy: true,
    WeakMap: true,
    
    MSApp: true,
    __REACT_DEVTOOLS_GLOBAL_HOOK__: true,
    
    
    
    module: true,
    define: true,
    require: true,
    global: true,
  },
  parserOptions: {
    ecmaVersion: 5,
    sourceType: 'script',
  },
  rules: {
    'no-undef': 'error',
    'no-shadow-restricted-names': 'error',
  },
};