go/parser: parses comma terminated fields

gotype successfully parses (via go/parser) the following program:

``` go
package a
type T struct{int,}
```

while gc says:

```
go.go:2: syntax error: unexpected }, expecting name
```

As far as I see, gc is right. I don't see where spec allows commas there.

on commit af817890aa7b628234075a6cb01a3a435fb8317d
