Call to a possibly undefined method asyncResponder

Hi, why would I be getting this error for this code

            token.addResponder(Async.asyncResponder(this, new AsyncResponder(onResult, faultHandler), 600));

1061: Call to a possibly undefined method asyncResponder through a reference with static type Class. 

my imports look fine

import flexunit.framework.Assert;
    import org.flexunit.async.Async;