Make dart2js_info work again after partial migration to null-safety

Change-Id: I09b87ce1d6978f2b8aed65ecefe55557dd7b5784
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/246581
Reviewed-by: Nate Biggs <natebiggs@google.com>
Reviewed-by: Alexander Thomas <athom@google.com>
Commit-Queue: Ilya Yanok <yanok@google.com>
diff --git a/pkg/dart2js_info/lib/info.dart b/pkg/dart2js_info/lib/info.dart
index 55c3c50..ef0c1a5 100644
--- a/pkg/dart2js_info/lib/info.dart
+++ b/pkg/dart2js_info/lib/info.dart
@@ -334,7 +334,7 @@
   late final bool isConst;
 
   /// When [isConst] is true, the constant initializer expression.
-  late final ConstantInfo initializer;
+  ConstantInfo? initializer;
 
   FieldInfo(
       {required String name,