Wrap NodeScript binding class in JavaScript layer

This makes it easy to prevent errors where Script methods
are called on non-script objects, resulting in Assertion failures.

lib/vm.js

@@ -21,12 +21,34 @@
 
 var binding = process.binding('evals');
 
-exports.Script = binding.NodeScript;
-exports.createScript = function(code, ctx, name) {
-  return new exports.Script(code, ctx, name);
+module.exports = Script;
+Script.Script = Script;
+
+function Script(code, ctx, filename) {
+  if (!(this instanceof Script)) {
+    return new Script(code, ctx, filename);
+  }
+
+  var ns = new binding.NodeScript(code, ctx, filename);
+
+  // bind all methods to this Script object
+  Object.keys(binding.NodeScript.prototype).forEach(function(f) {
+    if (typeof binding.NodeScript.prototype[f] === 'function') {
+      this[f] = function() {
+        if (!(this instanceof Script)) {
+          throw new TypeError('invalid call to '+f);
+        }
+        return ns[f].apply(ns, arguments);
+      };
+    }
+  }, this);
+};
+
+Script.createScript = function(code, ctx, name) {
+  return new Script(code, ctx, name);
 };
 
-exports.createContext = binding.NodeScript.createContext;
-exports.runInContext = binding.NodeScript.runInContext;
-exports.runInThisContext = binding.NodeScript.runInThisContext;
-exports.runInNewContext = binding.NodeScript.runInNewContext;
+Script.createContext = binding.NodeScript.createContext;
+Script.runInContext = binding.NodeScript.runInContext;
+Script.runInThisContext = binding.NodeScript.runInThisContext;
+Script.runInNewContext = binding.NodeScript.runInNewContext;