go/types: cyclical type declaration accepted w/o error

<pre>The program:

<a href="http://play.golang.org/p/HVo1iZeaiW">http://play.golang.org/p/HVo1iZeaiW</a>

is accepted by gotype w/o an error.

Both gc and gccgo complain.

$ gccgo x.go
x.go:3:9: error: array bound is not constant
 type T [len(a)]int
         ^</pre>
