Upstream at go.tools changed Context to Config.

The guys at go.tools just changed types.Context to be types.Config. So here's a simple fix.

see: http://code.google.com/p/go/source/detail?repo=tools&r=5a9c2a38449e3b024dd12d98e2f33e686d9f95a0&spec=svn.tools.e5e9ae03f5d14c1eb1b6d4982e3ad33c9470a299#publish