Cannot specify build flags to detect correct file for `rice embed-go`

I have two files:

js_asset_prod.go:

``` go
// +build production

package server

import (
"github.com/GeertJohan/go.rice"
)

var javascriptAssetRiceBox = rice.MustFindBox("./js/prod").HTTPBox() // <--- prod path
```

and js_asset_dev.go:

``` go
// +build !production

package server

import (
"github.com/GeertJohan/go.rice"
)

var javascriptAssetRiceBox = rice.MustFindBox("./js/dev").HTTPBox() // <-- dev path
```

And when I run `rice -v -i ./server/ embed-go` I get output like:

```
2015/05/06 23:50:17 skipping file "~/server/js-dev.rice-box.go"
2015/05/06 23:50:17 scanning file "~/server/js_asset_dev.go"
2015/05/06 23:50:17 found box "js/dev"
2015/05/06 23:50:17 scanning file "~/server/server.go"
2015/05/06 23:50:17
2015/05/06 23:50:17 embedding box 'js/dev'
2015/05/06 23:50:17 to file js-dev.rice-box.go
2015/05/06 23:50:17 includes dir: ''
2015/05/06 23:50:17
2015/05/06 23:50:17 rice finished successfully
```

It appears that rice does not recognize js_asset_prod.go because it has an opt-in build flag, and in go.rice/flags.go, it uses `build.ImportDir` which does not support adding build flags (and the rice CLI does not support adding build flags either).