This change does not actually use the token handle for GetToken right
now, but implements the approach for memoizing *token.Files.
Change-Id: I75919f4e97abd6893b202c021adecd2c9dbfc2be
Reviewed-on: https://go-review.googlesource.com/c/tools/+/182277
Run-TryBot: Rebecca Stambler <rstambler@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Cottrell <iancottrell@google.com>