code: function getMetaKeyCode(unknown) {
              var CTRL_MASK = 4096;
              var ALT_MASK = 8192;
              var SHIFT_MASK = 16384;
              var metaKeyCode = 0;
              if (typeof unknown == "string") {
                  // unknown as string
                  unknown.toLowerCase().split(/\+\s*/).forEach(function(name) {
                      switch (name) {
                        case "ctrl":
                        case "cmd":
                          metaKeyCode |= CTRL_MASK;
                          break;
  
                        case "alt":
                          metaKeyCode |= ALT_MASK;
                          break;
  
                        case "shift":
                          metaKeyCode |= SHIFT_MASK;
                          break;
  
                        default:
                          metaKeyCode |= keymap[name];
                      }
                  });
              } else {
                  // unknown as key event
                  if (unknown.ctrlKey || unknown.metaKey) {
                      metaKeyCode |= CTRL_MASK;
                  }
                  if (unknown.altKey) {
                      metaKeyCode |= ALT_MASK;
                  }
                  if (unknown.shiftKey) {
                      metaKeyCode |= SHIFT_MASK;
                  }
                  metaKeyCode |= unknown.keyCode;
              }
              return metaKeyCode;
          }
docs: 计算包含 meta 键的 keycode
  
  @param  {String|KeyEvent} unknown