[Repository] Item tree result are not correctly sorted in Chrome

[jc7447]

With Chrome, an item tree call may not place the selected item in first position.

This is because we are using javascript object as a Map, assuming that the keys order will be keep when iterating over it.

We should probably use ECMA script 6 map : https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Map