Access to instance members in async postconditions lead to invalid IL

[SergeyTeplyakov]

Consider following example:

class State
{
    private int _state;
    public async Task<int> StartAsync()
    {
        Contract.Ensures(Contract.Result<int>() != 0 && _state == -1);
        await Task.Delay(42);
    }
}

With current implmenetation AsyncClosure doesn't have a _this field that will point to enclosing class, that's why using any instance members in async postconditions lead to invalid IL.