No longer use vm to load code.

Improves performance 2x on node > 0.10.

Ref #636

lib/ast.js

@@ -81,6 +81,7 @@ function DEFNODE(type, props, methods, base) {
     ctor.DEFMETHOD = function(name, method) {
         this.prototype[name] = method;
     };
+    exports["AST_" + type] = ctor;
     return ctor;
 };