string.IndexOf(String value, StringComparison comparisonType) not available

IndexOf function with StringComparison pararameter is not available.
Here is test case:
```c#
using System;

public static class Program
{
    public static void Main(string[] args)
    {
        var s = "abcdefabcdef";
        Console.WriteLine(s.IndexOf("ABC", StringComparison.OrdinalIgnoreCase));
    }
}
```