Issue title: computed key marked duplicate
 Issue body: ```js
var x = "y";
var z = {
  x: x,
  [x]: x
};
```

```
  2:8  error  Duplicate key 'x'  no-dupe-keys

✖ 1 problem
```

is this by design?
 Comments: 
 Comment 0: In this context it's a bug, because you're creating a property called "y", so it's not a duplicate. If you did this it would be a valid error:

``` js
var z = {
    x: x,
   ["x"]: x
}
```

 Comment 1: Working on this.
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