Object.groupBy now builds its result record with a null prototype,
stored as a flag on the record's overflow map. Inspection prints
"[Object: null prototype]", Object.getPrototypeOf answers null, `in`
finds no inherited names, and String(), template literals, toString(),
hasOwnProperty() and propertyIsEnumerable() throw Node's TypeErrors.
Spread copies and structuredClone keep Object.prototype, as in Node.
Index-signature records converted from an Object.create(null) value
keep the null prototype too.
Chris Tate
844aa87ae0