fix AST_Node.optimize() (#1602)

Liberal use of `Compressor.transform()` and `AST_Node.optimize()` presents an issue for look-up operations like `TreeWalker.in_boolean_context()` and `TreeWalker.parent()`.

This is an incremental fix such that `AST_Node.optimize()` would now contain the correct stack information when called correctly.

lib/utils.js

@@ -126,9 +126,11 @@ function merge(obj, ext) {
     return count;
 };
 
-function noop() {};
+function noop() {}
 function return_false() { return false; }
 function return_true() { return true; }
+function return_this() { return this; }
+function return_null() { return null; }
 
 var MAP = (function(){
     function MAP(a, f, backwards) {