ac = (a, b) => {
    return (e, k) => {
      const action = (e.shiftKey || e.ctrlKey || e.altKey || e.metaKey ||
                      !this.keyboard.applicationCursor) ? a : b;
      return resolve(action, e, k);
    };
  }
If mod or not application cursor a, else b.  The keys that care about
application cursor ignore it when the key is modified.

@param {!hterm.Keyboard.KeyDefAction} a
@param {!hterm.Keyboard.KeyDefAction} b
@return {!hterm.Keyboard.KeyDefFunction}