How can I reinstall "Installer.app"?
This happened to me, too. I do not know the reason exactly, but I think it has something to do with the upgrade to the 10.10.3 beta.

My Installer.app (located in: /System/Library/CoreServices/ file had a circle on it at the file size was just 52 KB.

Luckily I had another Mac (running plain 10.10.2) around and could just copy the file over with a USB stick.