Fix issue with --unsafe-package-serialization

--unsafe-package-serialization saves the frontend_server from having to
re-serialize everything all the time.

Before this CL, though, things could go wrong:

If you had a situation where you had previously compiled a file that
depend on a package A, and where (some of) package A depends on
package B this would happen:
- All of package A was serialized together.
- All of package B was serialized together.

When later, you compile something that depend less on package A - namely
on only on parts that does not depend on package B, but is included in
the previously serialized package A, the following would happen:
- The new (non-package) libraries would be serialized.
- Package A serialization would be reused.

This is basically fine: Running the app would be fine, everything it
actually depend on is there.

However, if the VM is forced to compile everything it now also compiles
stuff that was included - but really unused - from package A - namely
also the libraries that depend on package B --- which is not included.

This CL changes the last part by also including package B.

The result is that even more unused libraries are included, but that
the VM can survive a forceful compile.

Note that the --unsafe-package-serialization is only used for tests,
so the "including even more unused" part is probably not a big deal,
and all in all there's still a big speed advantage to doing this.

Change-Id: Iac06ba6f40c2caaacce641c5853e9491496dce53
Reviewed-on: https://dart-review.googlesource.com/c/78541
Reviewed-by: Vyacheslav Egorov <vegorov@google.com>
Commit-Queue: Jens Johansen <jensj@google.com>