Import changes from std lib version, specifically
https://golang.org/cl/153821
which fixes a bug (#29198) in reading of import data.
Change-Id: I26d4da015b88bbc8f4f7775b0850bef5dffadbec
Reviewed-on: https://go-review.googlesource.com/c/153868
Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>