Symbol.for throws SyntaxError when called with no argument

[d01c2]

Build environment: macOS 26.5.2

Moddable SDK version:

• Installed via jsvu, so there is no SDK source tree / git describe.
• Engine reports: XS 17.9.3, slot 32 bytes, ID 4 bytes

Target device: desktop (host xs)

Description
The first step of Symbol.for is ToString(key). With no argument, key is undefined and ToString yields "undefined", so the registry is consulted for that key and a Symbol with description "undefined" is returned. XS rejects the call instead.

Steps to Reproduce

$ xst -e 'print(String(Symbol.for()));'
SyntaxError: no key (in Symbol.for)

Expected behavior
Symbol(undefined)

For comparison, V8 (15.6.20):

$ v8 -e 'print(String(Symbol.for()));'
Symbol(undefined)

Other information
Symbol.for has no step that distinguishes a missing argument from an explicit undefined, and nothing before ToString can raise a SyntaxError. JavaScriptCore, SpiderMonkey, GraalJS and QuickJS-ng all return Symbol(undefined), and all of them register it under the key "undefined", so Symbol.keyFor(Symbol.for()) is "undefined" and Symbol.for() === Symbol.for("undefined").