Issue #1988968 by nod_, droplet, bnjmnm, viappidu, extect, jansete, olli,...

Issue #1988968 by nod_, droplet, bnjmnm, viappidu, extect, jansete, olli, martin107, pmagunia, bartlangelaan, Wim Leers, zrpnr, KapilV, yogeshmpawar, Spleshka, Phil Wolstenholme, DuaelFr, agata.guc, alwaysworking, dawid_nawrot, andriic, keithdoyle9, gapple, lauriii, Martijn de Wit, jefuri, JMOmandown, larowlan, rubens.arjr, borisson_, joseph.olstad, jberube, gilgabar, Poindexterous, Aless86, jessebeach, bojanz, phma, aheimlich, heddn, phenaproxima, acbramley, codebymikey, cmlara, das-peter, matthiasm11, acolden, xjm, jrockowitz, pianomansam, clairemistry, John Pitcairn: Drupal.ajax does not guarantee that "add new JS file to page" commands have finished before calling said JS