[es6] Initial steps towards a correct implementation of IsCallable.

src/proxy.js

Index: src/proxy.js
diff --git a/src/proxy.js b/src/proxy.js
index c8ad42ff84aba88b8558406b4ecf1bf1101d22b6..fbca982d8ce881f5316271d19bdef8401d72f250 100644
--- a/src/proxy.js
+++ b/src/proxy.js
@@ -35,11 +35,11 @@ function ProxyCreate(handler, proto) {
 function ProxyCreateFunction(handler, callTrap, constructTrap) {
   if (!IS_SPEC_OBJECT(handler))
     throw MakeTypeError(kProxyHandlerNonObject, "createFunction")
-  if (!IS_SPEC_FUNCTION(callTrap))
+  if (!IS_CALLABLE(callTrap))
     throw MakeTypeError(kProxyTrapFunctionExpected, "call")
   if (IS_UNDEFINED(constructTrap)) {
     constructTrap = DerivedConstructTrap(callTrap)
-  } else if (IS_SPEC_FUNCTION(constructTrap)) {
+  } else if (IS_CALLABLE(constructTrap)) {
     // Make sure the trap receives 'undefined' as this.
     var construct = constructTrap
     constructTrap = function() {