Incorrect emit for a property assignment referencing a renamed let

``` ts
var x;
if (true) {
    let x;
    var obj1 = { x: x }; // Should be { x: _x }, emits as { _x: _x }
    var obj2 = { x }; // Should be { x: _x }, emits as { _x: x }
}
```

The problem is that we are doing this rename in emitIdentifier, and we are not being picky enough about where we rename. Property names should never be renamed, and identifier references should always be renamed.
