UpgradeAllProjectsToCSharp8_NullableReferenceType unit-test fails in src\EditorFeatures\CSharpTest\Diagnostics\UpgradeProject\UpgradeProjectTests.cs

[AlekseyTs]

        // PROTOTYPE(NullableReferenceTypes): Enable.
        [Fact(Skip = "TODO")]
        public async Task UpgradeAllProjectsToCSharp8_NullableReferenceType()
        {
            await TestLanguageVersionUpgradedAsync(
@"<Workspace>
    <Project Language=""C#"" LanguageVersion=""6"">
        <Document>
class C
{
    void A(string? [|s|])
    {
    }
}
        </Document>
    </Project>
    <Project Language=""C#"" LanguageVersion=""6"">
    </Project>
    <Project Language=""C#"" LanguageVersion=""Default"">
    </Project>
    <Project Language=""C#"" LanguageVersion=""7"">
    </Project>
    <Project Language=""Visual Basic"">
    </Project>
</Workspace>",
                LanguageVersion.CSharp8,
                parseOptions: null,
                index: 1);
        }