Locke's vision was not a glimpse of the future but a metaphorical situation, it was not meant to be taken literally. Boone was a guide, he was meant to show Locke that Eko was injured in the polar bear cave, and tell Locke he needed to clean up his mess. (i.e. causing the swan hatch to implode)

__________________
" We kinda like knew that forever ago." -Hurley SE4EP8