Analyzer: Fix null safety migration error in experimental flag generation
Analyzer: Fix null safety migration error in experimental flag generation

It's possible for an entry in `tools/experimental_features.yaml` to
lack an `expired` line, so we need to represent it as a `bool?`.

Avoids a crash when running `dart
pkg/analyzer/tool/experiments/generate.dart`.

Change-Id: I8734ccb9e6c5e43754b0ef870b08c304cc1470ca
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/182300
Reviewed-by: Konstantin Shcheglov <09e4d7516628963212bf4aace2f97603d2b706e4@google.com>
Commit-Queue: 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