reflect value.Interface not copy for struct

demo
```
package main

import (
	"fmt"
	"reflect"
)

type student struct {
	name string
}

func main() {
	a := []student{student{"A"}, student{"B"}, student{"C"}}
	v := reflect.ValueOf(a)
	i := v.Index(0).Interface()
	a[0] = student{"X"}
	fmt.Println(a, i)
}
```

go run main.go
```
[{X} {B} {C}] {A}
```

gopherjs run main.go
```
[{X} {B} {C}] {X}
```
