New feature: DEE - Check if dead expressions have never-assigned vars?

[jcrodriguez1989]

The optimizer is currently eliminating any dead expression, however, the code:

foo <- function() {
  x
  return(8818)
}

is not equivalent to

foo <- function() {
  return(8818)
}

Both functions will return the same value, however, the first one will give an error similar to:

foo()
## Error in foo() : object 'x' not found

[saxenism]

Hey @jcrodriguez1989 , please have a look here. It's a possible solution. Let me know how it can be improved further. And yeah, all the build tests are passing.