What steps will reproduce the problem?
1. create file "a" and "b"
2. run `os.Rename("a", "b")`

What is the expected output?
file "b" should be atomically replaced with file "a" 

What do you see instead?
rename a b: Cannot create a file when that file already exists.

Which compiler are you using (5g, 6g, 8g, gccgo)?
6g

Which operating system are you using?
windows 8 64-bit


Which version are you using?  (run 'go version')
go version go1.1rc3 windows/amd64


I think it should use MoveFileEx syscall with MOVEFILE_REPLACE_EXISTING flag set instead
of plain MoveFile syscall.