IAwaitable.cs

public interface IAwaitable<out TResult> : ICriticalNotifyCompletion
{
    bool IsCompleted { get; }
    TResult GetResult();
}

public static class AwaitableExtensions
{
    public static IAwaitable<TResult> GetAwaiter<TResult>(this IAwaitable<TResult> source)
    {
        return source;
    }
}