Migration: Pass ExpressionChecks to _checkAssignment instead of the raw expression.
Migration: Pass ExpressionChecks to _checkAssignment instead of the raw expression.

This paves the way for a follow-up CL that will introduce a more
general notion of the "origin" of an edge (of which ExpressionChecks
will be just one kind of origin).

Change-Id: I76ee12679d881ef95a4ba17a224fe85ecf6b589f
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/106380
Reviewed-by: Brian Wilkerson <1f7641b6b14c52b9163524ab8d9aabff80176f21@google.com>
Reviewed-by: Konstantin Shcheglov <09e4d7516628963212bf4aace2f97603d2b706e4@google.com>

dart-lang/sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dart-lang/sdk,dartino/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-lang/sdk