What kind of performance increases could we see if we simply added type annotations to Javascript? Would this give the VMs a significant boost?

Something like AS3 (Harmony) syntax:

  var foo:int = 3;
  var bar:string = "A nice string";
  var baz:Array = ["a", "polymorphous", 4, "array"];
  var v:Array.<int> = [2,3,4];

  function f(a:int, b:string):string {
    return a + " " + b;
  }

Sidenote: I really want someone to write a Coffeescript compiler that supports type annotations like the above. It can strip them out when it compiles the code; I just want it to yell at me if I violate one of my declared types.