This initial commit includes: the definitions of a modular test, a module, a
pipeline, and an in-memory implementation of such pipeline, and an IO implementation.
Change-Id: I69056342da8ba126459064d7751d5dbe75ebcfca
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/100627
Reviewed-by: Johnni Winther <johnniwinther@google.com>