This adds [Value] and [Property] classes the helps defined the
properties of the yaml files used for incremental test declaratively.
This also serves as natural point for documented the properties.
Change-Id: Iaa04ffb23608e592b8f0401d3d0fe4bb8fac5066
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/251549
Reviewed-by: Chloe Stefantsova <cstefantsova@google.com>
Commit-Queue: Johnni Winther <johnniwinther@google.com>