Homepage download links don't work

[alanjoyce]

Both the general download button in the header and the OS-specific ones at the bottom of the page don't work yet.

[adam-singer]

The download button fixed at the bottom at adam-singer@c757f53

What should the header download button point to at default. I guess the logic should be the same for the buttons at the bottom but the class needs one different attribute.

[adam-singer]

btw, this will contain merge conflicts when rebasing with the main dartlang site. Not bad todo, I got most of a rebase done today but had to abort it cause something might of broken in the main tree in terms of getting the version files.

[alanjoyce]

Bottom links look good.

For the header download button, let's follow the same logic as the buttons, so ideally it will default to the user's OS and change if the user changes it at the bottom.

[adam-singer]

fixed at: adam-singer@d5a7199