title: Eslint no-unused-var for decorator factory
body: <p>Thanks for browsing my question.<br>
  I met some eslint errors after using decorator like this</p>
  
  <pre><code>@decorate({ title: 'head' })
  class Test {
    @decorate(item =&gt; ({ age: item.age }))
    method(){
      return ;
    }
  }
  export default Test;
  </code></pre>
  
  <p>.eslintrc config :</p>
  
  <pre><code>{
    "parser": "babel-eslint",
    "rules": {
      "no-unused-vars": 2
    }
  }
  </code></pre>
  
  <p>pacakges version</p>
  
  <pre><code>"babel-eslint": "6.0.3",
  "eslint": "^2.8.0"
  </code></pre>
  
  <p>eslint error message</p>
  
  <pre><code>Cannot read property 'type' of undefined
  TypeError: Cannot read property 'type' of undefined
    at collectUnusedVariables (/Users/Documents/support_decorator/node_modules/eslint/lib/rules/no-unused-vars.js:184:44)
  </code></pre>
  
  <p>Looks like this line causes problem. And used ES5 common anonymous function, errors still exist.</p>
  
  <pre><code>@decorate(item =&gt; ({ age: item.age })) 
  </code></pre>
  
  <p>What should i do? Upgrade eslint and babel-eslint to specific version or just close "no-unused-vars" rule?</p>