Misplaced annotation warning with computed properties in ES6 classes

``` javascript
let bar = 'bar';
class Foo {
  /** @return {boolean} */
  get [bar]() { return true; }

  /** @return {boolean} */
  ['baz']() { return true; }
}
```

Both members get a:
`WARNING - Misplaced function annotation. This JSDoc is not attached to a function node. Are you missing parentheses?`

(run with `--language_in=ECMASCRIPT6_STRICT -O SIMPLE`)
