If the character after $ cannot be detected as a valid
variable declaration, do not gobble the $.

Fixes #24345

Change-Id: Iec47be1f2e4f8147b8ceb64c30778eae8045b58f
Reviewed-on: https://go-review.googlesource.com/103055
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>