Flow analysis: split interface and implementation, and add more doc comments.

No functional change.

Change-Id: I67fe2acdacdd68fa5841da087ff973885dc6325f
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/121772
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>

Author: stereotype441