FunctionInitialize does not take into account that "length", "caller" or "arguments" may be already present on the function object. To avoid undefined behaviour or for example strict-mode functions without poisoned "caller" properties etc., I'd suggest to:
(1) Make steps 12-13 the initial steps of FunctionInitialize
(2) Change [[DefineOwnProperty]] to DefinePropertyOrThrow()
This is similar to the String constructor which sets internal properties after user-modifiable properties.
Test case for strict-mode function object without poisoned "caller" property:
$ ./src/main/bin/repl.sh --strict
js> class Fn extends Function{ constructor(){} }
js> f = new Fn(), Object.defineProperty(f, "caller", {value: null})
function F() { /* source not available */ }
js> Function.call(f, "'use strict'")
function anonymous () {
'use strict'
}
js> Object.getOwnPropertyDescriptor(f, "caller")
{value:null,writable:false,enumerable:false,configurable:false}
fixed in rev 16 editor's draft
fixed in rev16 draft. July 15, 2013