Previously such shared code was in the front_end package, but that
created problems because there is a lot of code in front_end that
isn't intended to be shared with the analyzer (including, notably, the
dependency on kernel).
This CL just moves over the flow_analysis logic to the new shared
package. Follow-up CLs will move over other shared logic and tests.
The end goal is that the analyzer package will no longer have a
dependency on front_end.
Change-Id: I5642d6565204422d79808ca47648462db85e442a
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/123624
Reviewed-by: Jens Johansen <jensj@google.com>
