Flow types reported as unused in function params with destructuring.

[hzoo]

Ref babel/babel#3275

Example

import type NodePath from "../index";
var a = {
  checkPath({ node, parent }: NodePath): boolean {
  }
}

/babel/packages/babel-traverse/src/path/lib/virtual-types.js
   31:31  error  "NodePath" is defined but never used  no-unused-vars
   38:31  error  "NodePath" is defined but never used  no-unused-vars
  109:23  error  "NodePath" is defined but never used  no-unused-vars

[hzoo]

Closed by #246