What does 'go version' print?
go version devel +07e31caba5b6 Wed Apr 09 21:50:24 2014 -0700 + linux/amd64

What steps reproduce the problem?
Run this program. http://play.golang.org/p/uO-vW5gC2X

package main

import (
    "expvar"
    "fmt"
)

func main() {
    m := expvar.NewMap("aaa")
    m.Add(`"xxx`, 1)
    fmt.Println(m.String())  // prints {""xxx": 1}
}

What happened?
The program prints a malformed JSON string.

What should have happened instead?
Double quote characters should be escaped by backslash.