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

src/macros.py

Index: src/macros.py
diff --git a/src/macros.py b/src/macros.py
index 63b0b29489c28bb1df526154814120a01eb6a36c..baaea933abe48804370dc32c05980f07737651cf 100644
--- a/src/macros.py
+++ b/src/macros.py
@@ -128,10 +128,7 @@ macro IS_SPEC_OBJECT(arg)   = (%_IsSpecObject(arg));
 
 # Macro for ECMAScript 5 queries of the type:
 # "IsCallable(O)"
-# We assume here that this is the same as being either a function or a function
-# proxy. That ignores host objects with [[Call]] methods, but in most situations
-# we cannot handle those anyway.
-macro IS_SPEC_FUNCTION(arg) = (%_ClassOf(arg) === 'Function');
+macro IS_CALLABLE(arg) = (typeof(arg) === 'function');
 
 # Macro for ES6 CheckObjectCoercible
 # Will throw a TypeError of the form "[functionName] called on null or undefined".