function shouldThrow(func, errorMessage) {
    var errorThrown = false;
    var error = null;
    try {
        func();
    } catch (e) {
        errorThrown = true;
        error = e;
    }
    if (!errorThrown)
        throw new Error('not thrown');
    if (String(error) !== errorMessage)
        throw new Error(`bad error: ${String(error)}`);
}

shouldThrow(function () {
    function *fib(flag)
    {
        let a = 1;
        let b = 1;
        yield 42;
        if (flag)
            return c;
        let c = 500;
    }

    let value = 0;
    for (let i = 0; i < 1e4; ++i) {
        for (let v of fib(false)) {
        }
    }
    for (let v of fib(true)) {
    }
}, `ReferenceError: Cannot access uninitialized variable.`);