go/loader: only run examples on 1.8
go/loader: only run examples on 1.8

Tip introduces a new internal/poll package that breaks the expected
output.

Fixes golang/go#19150
Updates golang/go#19152

Change-Id: I5ff7e8a92afe4d25feb6365933062e931c9b435f
Reviewed-on: https://go-review.googlesource.com/37148
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
Reviewed-by: Alan Donovan <7069e9552f9d110b0a0f326b96f22f63fb99dfc1@google.com>

syreclabs/go-tools,syreclabs/go-tools,syreclabs/go-tools,golang/tools,golang/tools,golang/tools,golang/tools,syreclabs/go-tools,golang/tools,syreclabs/go-tools,syreclabs/go-tools