Make function proxies work as constructors.

src/proxy.js

Index: src/proxy.js
diff --git a/src/proxy.js b/src/proxy.js
index 123e5671ac90727328b6bd5bd71d4fdbac068006..8162a3903db00e625b688b1ffb16e862634eb9b5 100644
--- a/src/proxy.js
+++ b/src/proxy.js
@@ -42,13 +42,18 @@ $Proxy.createFunction = function(handler, callTrap, constructTrap) {
   if (!IS_FUNCTION(callTrap))
     throw MakeTypeError("trap_function_expected", ["createFunction", "call"])
   if (IS_UNDEFINED(constructTrap)) {
-    constructTrap = callTrap
-  } else if (!IS_FUNCTION(constructTrap)) {
+    var construct = DerivedConstructTrap(callTrap)
+  } else if (IS_FUNCTION(constructTrap)) {
+    construct = function() {
+      // Make sure the trap gets undefined as this.
+      return $Function.prototype.apply.call(constructTrap, void 0, arguments)
+    }
+  } else {
     throw MakeTypeError("trap_function_expected",
                         ["createFunction", "construct"])
   }
   return %CreateJSFunctionProxy(
-    handler, callTrap, constructTrap, $Function.prototype)
+    handler, callTrap, construct, $Function.prototype)
 }
 
 
@@ -57,13 +62,22 @@ $Proxy.createFunction = function(handler, callTrap, constructTrap) {
 // Builtins
 ////////////////////////////////////////////////////////////////////////////////
 
+function DerivedConstructTrap(callTrap) {
+  return function() {
+    var proto = this.prototype
+    if (!IS_SPEC_OBJECT(proto)) proto = $Object.prototype
+    var obj = new $Object()
+    obj.__proto__ = proto
+    var result = $Function.prototype.apply.call(callTrap, obj, arguments)
+    return IS_SPEC_OBJECT(result) ? result : obj
+  }
+}
+
 function DelegateCallAndConstruct(callTrap, constructTrap) {
   return function() {
     if (%_IsConstructCall()) {
-      // return constructTrap.apply(this, arguments)
       return $Function.prototype.apply.call(constructTrap, this, arguments)
     } else {
-      // return callTrap.apply(this, arguments)
       return $Function.prototype.apply.call(callTrap, this, arguments)
     }
   }