Aims to automate the process of bumping a single dependency.
To bump pub_semver to the newest commit invoke as:
```
> dart tool/manage_deps.dart bump pub_semver
```
Change-Id: I16a3edabfb03adbd4460fb3cd81738a792e44d50
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/213764
Commit-Queue: Sigurd Meldgaard <sigurdm@google.com>
Reviewed-by: Alexander Thomas <athom@google.com>
Reviewed-by: Jonas Jensen <jonasfj@google.com>
