Existence checks are missing in switch/pattern matching when using that

This code:

-> | foo? => 42

Produces this compiled output:

// Generated by LiveScript 1.4.0
(function(){
  switch (false) {
  case typeof foo == 'undefined' || foo === null:
    return 42;
  }
});

However, if you mention that in the return clause:

-> | foo? => that

It will compile to this:

// Generated by LiveScript 1.4.0
(function(){
  var that;
  switch (false) {
  case (that = foo) == null:
    return that;
  }
});

Note missing checks for foo existence, so this code will die with ReferenceError when foo is not defined, and it shouldn't happen.

Indeed this can be seen even more easily like this:

if a?
    console.log that # => ReferenceError

PR #895 might fix this.

if a?
    console.log that

var that;
if (typeof a != 'undefined' && a !== null && (that = a, true)) {
  console.log(that);
}

if not a?
    console.log that

var that;
if ((that = undefined) || typeof a == 'undefined' || a === null) {
  console.log(that);
}

a = 10
if a?
    console.log that

var a;
a = 10;
if ((that = a) != null) {
  console.log(that);
}

a = 10
if not a?
    console.log that

var a;
a = 10;
if ((that = a) == null) {
  console.log(that);
}

The switch and while variants should work as well, and forms such as

if a? and not b?
    that

work the same as before I believe.

Any other cases to think about? :)

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