function onUnFold(cm, from, to) {
        var state = cm.state.foldGutter;
        var vp = cm.getViewport();
        delete cm._lineFolds[from.line];
        updateFoldInfo(cm, from.line, to.line || vp.to);
    }
Triggered when a folded code segment is unfolded.
@param {!CodeMirror} cm the CodeMirror instance for the active editor
@param {!{line:number, ch:number}} from  the ch and line position that designates the start of the region
@param {!{line:number, ch:number}} to the ch and line position that designates the end of the region