How can I reinstall "Installer.app"? When I double-click "Installer.app" in Macintosh HD ▸ System ▸ Library ▸ CoreServices, a dialog pops up that says "You can’t open the application “Installer.app” because it may be damaged or incomplete."

Also, the icon for "Installer.app" has a white circle over it.

I have no idea how this happened.

OS X Yosemite version 10.10.2
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.