Function joinClasses

static/editor.md/lib/codemirror/lib/codemirror.js:8205–8210

(a, b)

    if (!classTest(cls).test(current)) node.className += (current ? " " : "") + cls;
  };
  function joinClasses(a, b) {
    var as = a.split(" ");
    for (var i = 0; i < as.length; i++)
      if (as[i] && !classTest(as[i]).test(b)) b += " " + as[i];
    return b;
  }

  // WINDOW-WIDE EVENTS