function _setIn(treeData, objectPath, newValue) {
        return treeData.updateIn(objectPath, function (oldValue) {
            return newValue;
        });
    }
@private

The Immutable package does not have a `setIn` method, which is what this effectively
provides. This is a simple function that does an `updateIn` on treeData, replacing
the current value with the new one.

@param {Immutable.Map} treeData
@param {Array.<string>} objectPath path to object that should be replaced
@param {Immutable.Map} newValue new value to provide at that path
@return {Immutable.Map} updated treeData