update to dart dev 1.6.0-dev.9.5

Our build process only generates sha256 and md5 files. According to https://github.com/Homebrew/homebrew/wiki/Formula-Cookbook#updating-formulae homebrew supports sha256, unless I misread it?

I have opened a bug for dartlang.org to display sha256 on its download page. It won't appear immediately (some people are on vacation) but we have it queued up.

Will that work?