go.tools/cmd/vet: use new Assertable predicate (fix build)
go.tools/cmd/vet: use new Assertable predicate (fix build)

LGTM=r
R=r
CC=golang-codereviews
https://codereview.appspot.com/59210045

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