Migration: change EdgeBuilder to create flow analysis for each variable declaration.
Migration: change EdgeBuilder to create flow analysis for each variable declaration.

There's no good reason to unify the flow analysis for several
variables declared together, and it would break some assertions I'm
going to add to flow analysis in future CLs.

Change-Id: I4dafb7b5e9b078cb741d506cd0bbfab1bcab5ebb
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/120840
Reviewed-by: Konstantin Shcheglov <09e4d7516628963212bf4aace2f97603d2b706e4@google.com>

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