alt = (a, b) => {
    return (e, k) => {
      const action = !e.altKey ? a : b;
      return resolve(action, e, k);
    };
  }
If not e.altKey a, else b.

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