Implement function proxies (except for their use as constructors).

src/messages.js

Index: src/messages.js
diff --git a/src/messages.js b/src/messages.js
index 6603185a3207bf330bf4c1a34d04de782bdbc5cd..8f9dcaaf6a9ce30cdab55f36a26c3c55bf2e139b 100644
--- a/src/messages.js
+++ b/src/messages.js
@@ -194,6 +194,7 @@ function FormatMessage(message) {
       define_disallowed:            ["Cannot define property:", "%0", ", object is not extensible."],
       non_extensible_proto:         ["%0", " is not extensible"],
       handler_non_object:           ["Proxy.", "%0", " called with non-object as handler"],
+      trap_function_expected:       ["Proxy.", "%0", " called with non-function for ", "%1", " trap"],
       handler_trap_missing:         ["Proxy handler ", "%0", " has no '", "%1", "' trap"],
       handler_trap_must_be_callable: ["Proxy handler ", "%0", " has non-callable '", "%1", "' trap"],
       handler_returned_false:       ["Proxy handler ", "%0", " returned false for '", "%1", "' trap"],