go.tools/go/types: test case for issue 6413
go.tools/go/types: test case for issue 6413

Fixes issue 6413.

R=adonovan
CC=golang-dev
https://codereview.appspot.com/13840047

skyportsystems/go.tools,d0f/go-zh.tools,skyportsystems/go.tools,bryanxu/go-zh.tools,bryanxu/go-zh.tools,8limbs/gotools,d0f/go-zh.tools,webfd/go-zh.tools,d0f/go-zh.tools,bryanxu/go-zh.tools,d0f/go-zh.tools,8limbs/gotools,webfd/go-zh.tools,8limbs/gotools,bryanxu/go-zh.tools,webfd/go-zh.tools,skyportsystems/go.tools,skyportsystems/go.tools,webfd/go-zh.tools,skyportsystems/go.tools,webfd/go-zh.tools,d0f/go-zh.tools,bryanxu/go-zh.tools