Repair Disk seems to be no longer necessary at Apple announced, it therefore is not available in Disk Utility any more. However, we can still use Repair Disk function though Terminal.
Launch Terminal, then enter the following command line.
diskutil verifyvolume /Volumes/[the name of Disk you want to repair]/
Otherwise, you can bring back Disk Utility in Yosemite 10.10 to El Capitan. Read more...