interface型でない変数の型アサーション ref: http://qiita.com/kokardy/items/d3be8f8081f1a4438cc7

file0.go

t, ok := x.(T)

file1.go

type Tester interface {
    Test()
}

type A struct{}

func (a A) Test() {
    fmt.Println("called A.Test()")
}

func main() {
    a := new(A)
    tester, ok := interface{}(a).(Tester)
    //tester, ok := a.(Tester) //ERROR!
    fmt.Println("a is a Tester:", ok)
    tester.Test()
}