Updates for minor changes to the go/types API.
Updates for minor changes to the go/types API.

The type of go/types.Importer has changed from a func to an interface.
Make appropriate changes.

Maintain behavior of setting importError in Compile. This is needed to
maintain the fix for https://github.com/gopherjs/gopherjs/issues/119.

gopherjs/gopherjs,bpowers/gopherjs,gopherjs/gopherjs,bpowers/gopherjs,gopherjs/gopherjs,bpowers/gopherjs,gopherjs/gopherjs,jargv/gopherjs,gopherjs/gopherjs,jargv/gopherjs,albrow/gopherjs,albrow/gopherjs,jargv/gopherjs,albrow/gopherjs