Add a class to represent references tracked by flow analysis.

The only references that are promotable are references to variables,
however in the near future I'll be adding logic to flor analysis to
keep track of attempts to promote fields; this will require tracking
references to property gets as well as references to `this`, so we
need to start building up data structures to track those references.

Bug: #38773
Change-Id: I0e3fd44f580bb85db3e8f405675b66aa4069e455
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/179280
Commit-Queue: Paul Berry <[email protected]>
Reviewed-by: Konstantin Shcheglov <[email protected]>
Reviewed-by: Johnni Winther <[email protected]>

Author: stereotype441