function collapse(crumb, dontCoalesce)
        {
            if (crumb.classList.contains("hidden"))
                return;
            crumb.classList.add("collapsed");
            crumb.classList.remove("compact");
            if (!dontCoalesce)
                coalesceCollapsedCrumbs();
        }
@param {!Element} crumb
@param {boolean=} dontCoalesce