解决 Unexpected lexical declaration in case block 的问题

switch (2) {
  case 1:
    function f () {console.log('Wanna a girl friend !')}
    break
  case 2:
    f() // 会输出：Wanna a girl friend !
    break
}

switch (1) {
  case 1:
    let foo = 1
    break
  case 2:
    let foo = 2
    console.log(foo) // 会报错：Uncaught SyntaxError: Identifier 'foo' has already been declared
    break
}

switch (1) {
  case 1: {
    // do something
    break
  }
}