go.tools/importer: retain scope information.
go.tools/importer: retain scope information.

(It's needed by the oracle.)

R=crawshaw, gri
CC=golang-dev
https://codereview.appspot.com/13071043

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