function FileTreeViewModel() {
        // For convenience in callbacks, make a bound version of this method so that we can
        // just refer to it as this._commit when passing in a callback.
        this._commit = this._commit.bind(this);
    }
@constructor

Contains the treeData used to generate the file tree and methods used to update that
treeData.

Instances dispatch the following events:
- "change" (FileTreeViewModel.EVENT_CHANGE constant): Fired any time there's a change that should be reflected in the view.