Flow analysis: move tracking of definite assignment into VariableModel class.

Change-Id: I28cdc6f65aa3488da3538a7b920dc10f87ec1de0
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/115003
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>

Author: stereotype441