function _validateEdge(vertices, frontier, start, end) {
    var e0 = vertices[start].vertex;
    var e1 = vertices[end].vertex;
    for (var i = 0, len = frontier.length; i < len; i++) {
        var edge = frontier[i];
        if (!edge) continue;
        var v0 = vertices[edge[0]].vertex;
        var v1 = vertices[edge[1]].vertex;
        if ((e0 === v0 && (e1 === v1)) || (e0 === v1 && (e1 === v0))) {
            frontier[i] = null;
            return;
        }
    }
    frontier.push([start, end]);
}
Adds edge if not already on the frontier, removes if the edge or its reverse are on the frontier.
Used when reshaping DynamicGeometry's.

@method
@private
@param {Object[]} vertices Vec3 reference array.
@param {Array.<Number[]>} frontier Current edges potentially separating the features to remove from the persistant shape.
@param {Number} start The index of the starting Vec3 on the edge.
@param {Number} end The index of the culminating Vec3.
@return {undefined} undefined