Improve flow typing support for closures, instanceof and terminal blocks

[eric-milles]

Flow typing support is very basic at the moment. As the inference engine moves through the statements of a block, variable assignments overwrite the inferred type of the assigned variable. This does not consider conditional paths like nested closure blocks or blocks that return/throw.

def m1(def x) {
  def cl = { ->
    x = 1
  }
  x // type remains Object because Closure is not executed
}

def m2(def x) {
  if (predicate()) {
    x = 1
    return 0
   }
  x // type remains Object because conditional block does not share path here
}

def m3(def x) {
  if (x instanceof CharSequence) {
    x = Integer.valueOf(x)
  }
  x // bug prevents assignment within instanceof guard from propagating type info
}