Opt package:meta in to null safety.

This is done by setting the SDK constraint min version in the package's
pubspec to 2.9.0 which is the version that CFE currently uses to mean
"opt in".

However, the pre-built SDK currently checked into the repo can't handle
that version. To work around that, this CL has a gross hack to simply
omit the language version in the generated package config if the
pubspect's version is 2.9. This has the effect of telling CFE to use
the current language version, which is what we want, while also allowing
us to put the right SDK constraint in the package's pubspec.

Change-Id: I51c2952003c83b74fb6f3344c93839fbd267a3ec
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/145020
Auto-Submit: Bob Nystrom <rnystrom@google.com>
Commit-Queue: Leaf Petersen <leafp@google.com>
Reviewed-by: Leaf Petersen <leafp@google.com>