Here's a work-around for anyone who wants the old behaviour:
public static AndConstraint<GenericCollectionAssertions<T>> ReallyOnlyContain<T>(this GenericCollectionAssertions<T> itemsAssertion, Expression<Func<T, bool>> expectedItem)
{
var notExpectedItem = Expression.Lambda<Func<T, bool>>(Expression.Not(expectedItem.Body), expectedItem.Parameters);
return itemsAssertion.NotContain(notExpectedItem);
}
