[dart2js] Migrate kernel strategy
[dart2js] Migrate kernel strategy

Change-Id: Ib8b251bc69e42c414e6519595af75c10187abbef
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/264340
Reviewed-by: Nate Biggs <4bdbe5a28604b503f0b7346ee4d58c334975887b@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk