A dot-import cannot possibly introduce a `len` function since that
function would not be exported (it's lowercase). Furthermore, the
existing code already (incorrectly) assumed that there was no other
`len` function in another file of the package. Since this has been
an ok assumption for years, let's leave it, but remove the dot-import
restriction.

Fixes #15153.

Change-Id: I18fbb27acc5a5668833b4b4aead0cca540862b52
Reviewed-on: https://go-review.googlesource.com/21613Reviewed-by: Alan Donovan <adonovan@google.com>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>