better support for multiple input files:

- use a single AST_Toplevel node for all files
- keep original source filename in the tokens

lib/scope.js

@@ -188,6 +188,7 @@ AST_Toplevel.DEFMETHOD("figure_out_scope", function(){
                 } else {
                     g = new SymbolDef(self, node);
                     g.undeclared = true;
+                    globals[name] = g;
                 }
                 node.thedef = g;
                 if (name == "eval") {