use symbol value as key

[djfos]

What problem does this feature solve?

[Vue warn]: Avoid using non-primitive value as key, use string/number value instead.

symbol is primitive value and very suitable as key.

What does the proposed API look like?

no warnings with symbol

[posva]

The funny thing is the warning is technically incorrect because Symbol is a primitive type.

If we allow Symbols as keys, some people would try to use them as Symbol(aValue) and if that is done inside of a computed property, it would change every time the array changes (as Symbol always returns a new Symbol). So I think it would be worthy to also add a not on the docs.
Because after all, it's up to people to correctly use Symbols and most people will still use plain string or numbers id

[ElVisPL]

This warning still exists on IE11