Clarify migration guidance
Clarify migration guidance

Closes https://github.com/dart-lang/sdk/pull/42172
https://github.com/dart-lang/sdk/pull/42172

GitOrigin-RevId: af666ade7a03affc5210ddd644ac1f9ede682df1
Change-Id: I2cf627aab3d79c32025a2fd345d30dc97ef0cc02
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/149802
Reviewed-by: Kevin Moore <1d6e7d10d051cf619c5282da3c28fd8d3e0a65a4@google.com>
Reviewed-by: Paul Berry <72d414e3a99fb120308437779d51667bcc0984eb@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