[ package:vm_service ] Fix analysis error from https://dart.googlesource.com/sdk/+/fb5ffc827580ae49cd4611d3ddf0ad3c7c592c54

TEST=N/A

Change-Id: Iaa70b98d4918c9cfd38c32e197411318bb2cf7ac
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/178301
Reviewed-by: Ben Konyi <bkonyi@google.com>
diff --git a/pkg/vm_service/tool/dart/generate_dart.dart b/pkg/vm_service/tool/dart/generate_dart.dart
index 95a57cb..2e5743c6 100644
--- a/pkg/vm_service/tool/dart/generate_dart.dart
+++ b/pkg/vm_service/tool/dart/generate_dart.dart
@@ -1457,7 +1457,7 @@
         field.generateNamedParameter(gen, fromParent: fromParent);
       });
       if (hasRequiredParentFields) {
-        superType!.fields.where((field) => !field.optional).forEach(
+        superType.fields.where((field) => !field.optional).forEach(
             (field) => field.generateNamedParameter(gen, fromParent: true));
       }
       fields
@@ -1468,7 +1468,7 @@
     gen.write(')');
     if (hasRequiredParentFields) {
       gen.write(' : super(');
-      superType!.fields.where((field) => !field.optional).forEach((field) {
+      superType.fields.where((field) => !field.optional).forEach((field) {
         String? name = field.generatableName;
         gen.writeln('$name: $name,');
       });