minifier: should not remove `using a =` even if `a` is not used unless the value is known to not have `Symbol.dispose`

[sapphi-red]

{
    using a = { [Symbol.dispose]() {console.log("foo")} };
}

This should not be replaced with

{ [Symbol.dispose]() {console.log("foo")} };

because console.log("foo") should be called.
playground