r - 同じ結果を得るために、tapply を集計に変更できますか?

Question

dat=data.frame(
    year=c(rep(2007,5),rep(2008,3),rep(2009,3)),
    province=c("a","a","b","c","d","a","c","d","b","c","d"),
    sale=1:11)
tapply(dat$sale,list(dat$year,dat$province),sum)
      a  b  c  d
2007  3  3  4  5
2008  6 NA  7  8
2009 NA  9 10 11

この場合、tapply を集計に変更して同じ結果を得るにはどうすればよいですか?