[cfe] Focus nullable subtype error messages

Now that the types in the message were recently adjusted to include the '?', I think it's beneficial to focus on what the underlying problem is rather than the syntax.

Change-Id: I7e59ea3cc2ce618fe46052a9c5e04a973a5f6ab9
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/349581
Auto-Submit: Parker Lougheed <parlough@gmail.com>
Reviewed-by: Jens Johansen <jensj@google.com>
Reviewed-by: Chloe Stefantsova <cstefantsova@google.com>
Commit-Queue: Chloe Stefantsova <cstefantsova@google.com>

pkg/_fe_analyzer_shared/lib/src/messages/codes_generated.dart

@@ -13604,8 +13604,8 @@ Message _withArgumentsNullSafetyOptOutImplicit(String string) {
 const Template<Message Function(String name)> templateNullableInterfaceError =
     const Template<Message Function(String name)>(
   "NullableInterfaceError",
-  problemMessageTemplate:
-      r"""Can't implement '#name' because it's marked with '?'.""",
+  problemMessageTemplate: r"""Can't implement '#name' because it's nullable.""",
+  correctionMessageTemplate: r"""Try removing the question mark.""",
   withArguments: _withArgumentsNullableInterfaceError,
 );
 
@@ -13621,8 +13621,8 @@ Message _withArgumentsNullableInterfaceError(String name) {
   name = demangleMixinApplicationName(name);
   return new Message(
     codeNullableInterfaceError,
-    problemMessage:
-        """Can't implement '${name}' because it's marked with '?'.""",
+    problemMessage: """Can't implement '${name}' because it's nullable.""",
+    correctionMessage: """Try removing the question mark.""",
     arguments: {
       'name': name,
     },
@@ -13633,8 +13633,8 @@ Message _withArgumentsNullableInterfaceError(String name) {
 const Template<Message Function(String name)> templateNullableMixinError =
     const Template<Message Function(String name)>(
   "NullableMixinError",
-  problemMessageTemplate:
-      r"""Can't mix '#name' in because it's marked with '?'.""",
+  problemMessageTemplate: r"""Can't mix '#name' in because it's nullable.""",
+  correctionMessageTemplate: r"""Try removing the question mark.""",
   withArguments: _withArgumentsNullableMixinError,
 );
 
@@ -13650,7 +13650,8 @@ Message _withArgumentsNullableMixinError(String name) {
   name = demangleMixinApplicationName(name);
   return new Message(
     codeNullableMixinError,
-    problemMessage: """Can't mix '${name}' in because it's marked with '?'.""",
+    problemMessage: """Can't mix '${name}' in because it's nullable.""",
+    correctionMessage: """Try removing the question mark.""",
     arguments: {
       'name': name,
     },
@@ -13671,8 +13672,8 @@ const MessageCode messageNullableSpreadError = const MessageCode(
 const Template<Message Function(String name)> templateNullableSuperclassError =
     const Template<Message Function(String name)>(
   "NullableSuperclassError",
-  problemMessageTemplate:
-      r"""Can't extend '#name' because it's marked with '?'.""",
+  problemMessageTemplate: r"""Can't extend '#name' because it's nullable.""",
+  correctionMessageTemplate: r"""Try removing the question mark.""",
   withArguments: _withArgumentsNullableSuperclassError,
 );
 
@@ -13688,7 +13689,8 @@ Message _withArgumentsNullableSuperclassError(String name) {
   name = demangleMixinApplicationName(name);
   return new Message(
     codeNullableSuperclassError,
-    problemMessage: """Can't extend '${name}' because it's marked with '?'.""",
+    problemMessage: """Can't extend '${name}' because it's nullable.""",
+    correctionMessage: """Try removing the question mark.""",
     arguments: {
       'name': name,
     },

pkg/front_end/messages.yaml

@@ -5767,13 +5767,16 @@ AwaitInLateLocalInitializer:
   problemMessage: "`await` expressions are not supported in late local initializers."
 
 NullableSuperclassError:
-  problemMessage: "Can't extend '#name' because it's marked with '?'."
+  problemMessage: "Can't extend '#name' because it's nullable."
+  correctionMessage: "Try removing the question mark."
 
 NullableInterfaceError:
-  problemMessage: "Can't implement '#name' because it's marked with '?'."
+  problemMessage: "Can't implement '#name' because it's nullable."
+  correctionMessage: "Try removing the question mark."
 
 NullableMixinError:
-  problemMessage: "Can't mix '#name' in because it's marked with '?'."
+  problemMessage: "Can't mix '#name' in because it's nullable."
+  correctionMessage: "Try removing the question mark."
 
 JsInteropDartClassExtendsJSClass:
   problemMessage: "Dart class '#name' cannot extend JS interop class '#name2'."

pkg/front_end/testcases/enhanced_enums/simple_interfaces.dart.strong.expect

@@ -2,7 +2,8 @@ library;
 //
 // Problems in library:
 //
-// pkg/front_end/testcases/enhanced_enums/simple_interfaces.dart:21:20: Error: Can't implement 'I?' because it's marked with '?'.
+// pkg/front_end/testcases/enhanced_enums/simple_interfaces.dart:21:20: Error: Can't implement 'I?' because it's nullable.
+// Try removing the question mark.
 // enum E3 implements I? { // Error.
 //                    ^^
 //

pkg/front_end/testcases/enhanced_enums/simple_interfaces.dart.strong.transformed.expect

@@ -2,7 +2,8 @@ library;
 //
 // Problems in library:
 //
-// pkg/front_end/testcases/enhanced_enums/simple_interfaces.dart:21:20: Error: Can't implement 'I?' because it's marked with '?'.
+// pkg/front_end/testcases/enhanced_enums/simple_interfaces.dart:21:20: Error: Can't implement 'I?' because it's nullable.
+// Try removing the question mark.
 // enum E3 implements I? { // Error.
 //                    ^^
 //

pkg/front_end/testcases/enhanced_enums/simple_interfaces.dart.weak.expect

@@ -2,7 +2,8 @@ library;
 //
 // Problems in library:
 //
-// pkg/front_end/testcases/enhanced_enums/simple_interfaces.dart:21:20: Error: Can't implement 'I?' because it's marked with '?'.
+// pkg/front_end/testcases/enhanced_enums/simple_interfaces.dart:21:20: Error: Can't implement 'I?' because it's nullable.
+// Try removing the question mark.
 // enum E3 implements I? { // Error.
 //                    ^^
 //

pkg/front_end/testcases/enhanced_enums/simple_interfaces.dart.weak.modular.expect

@@ -2,7 +2,8 @@ library;
 //
 // Problems in library:
 //
-// pkg/front_end/testcases/enhanced_enums/simple_interfaces.dart:21:20: Error: Can't implement 'I?' because it's marked with '?'.
+// pkg/front_end/testcases/enhanced_enums/simple_interfaces.dart:21:20: Error: Can't implement 'I?' because it's nullable.
+// Try removing the question mark.
 // enum E3 implements I? { // Error.
 //                    ^^
 //

pkg/front_end/testcases/enhanced_enums/simple_interfaces.dart.weak.outline.expect

@@ -2,7 +2,8 @@ library;
 //
 // Problems in library:
 //
-// pkg/front_end/testcases/enhanced_enums/simple_interfaces.dart:21:20: Error: Can't implement 'I?' because it's marked with '?'.
+// pkg/front_end/testcases/enhanced_enums/simple_interfaces.dart:21:20: Error: Can't implement 'I?' because it's nullable.
+// Try removing the question mark.
 // enum E3 implements I? { // Error.
 //                    ^^
 //

pkg/front_end/testcases/enhanced_enums/simple_interfaces.dart.weak.transformed.expect

@@ -2,7 +2,8 @@ library;
 //
 // Problems in library:
 //
-// pkg/front_end/testcases/enhanced_enums/simple_interfaces.dart:21:20: Error: Can't implement 'I?' because it's marked with '?'.
+// pkg/front_end/testcases/enhanced_enums/simple_interfaces.dart:21:20: Error: Can't implement 'I?' because it's nullable.
+// Try removing the question mark.
 // enum E3 implements I? { // Error.
 //                    ^^
 //

pkg/front_end/testcases/extension_types/issue53840.dart.strong.expect

@@ -19,7 +19,8 @@ library;
 // extension type E6(Null it) implements dynamic {} // Error.
 //                                       ^
 //
-// pkg/front_end/testcases/extension_types/issue53840.dart:22:39: Error: Can't implement 'double?' because it's marked with '?'.
+// pkg/front_end/testcases/extension_types/issue53840.dart:22:39: Error: Can't implement 'double?' because it's nullable.
+// Try removing the question mark.
 // extension type E7(Null it) implements double? {} // Error.
 //                                       ^
 //

pkg/front_end/testcases/extension_types/issue53840.dart.strong.transformed.expect

@@ -19,7 +19,8 @@ library;
 // extension type E6(Null it) implements dynamic {} // Error.
 //                                       ^
 //
-// pkg/front_end/testcases/extension_types/issue53840.dart:22:39: Error: Can't implement 'double?' because it's marked with '?'.
+// pkg/front_end/testcases/extension_types/issue53840.dart:22:39: Error: Can't implement 'double?' because it's nullable.
+// Try removing the question mark.
 // extension type E7(Null it) implements double? {} // Error.
 //                                       ^
 //

pkg/front_end/testcases/extension_types/issue53840.dart.weak.expect

@@ -19,7 +19,8 @@ library;
 // extension type E6(Null it) implements dynamic {} // Error.
 //                                       ^
 //
-// pkg/front_end/testcases/extension_types/issue53840.dart:22:39: Error: Can't implement 'double?' because it's marked with '?'.
+// pkg/front_end/testcases/extension_types/issue53840.dart:22:39: Error: Can't implement 'double?' because it's nullable.
+// Try removing the question mark.
 // extension type E7(Null it) implements double? {} // Error.
 //                                       ^
 //

pkg/front_end/testcases/extension_types/issue53840.dart.weak.modular.expect

@@ -19,7 +19,8 @@ library;
 // extension type E6(Null it) implements dynamic {} // Error.
 //                                       ^
 //
-// pkg/front_end/testcases/extension_types/issue53840.dart:22:39: Error: Can't implement 'double?' because it's marked with '?'.
+// pkg/front_end/testcases/extension_types/issue53840.dart:22:39: Error: Can't implement 'double?' because it's nullable.
+// Try removing the question mark.
 // extension type E7(Null it) implements double? {} // Error.
 //                                       ^
 //

pkg/front_end/testcases/extension_types/issue53840.dart.weak.outline.expect

@@ -19,7 +19,8 @@ library;
 // extension type E6(Null it) implements dynamic {} // Error.
 //                                       ^
 //
-// pkg/front_end/testcases/extension_types/issue53840.dart:22:39: Error: Can't implement 'double?' because it's marked with '?'.
+// pkg/front_end/testcases/extension_types/issue53840.dart:22:39: Error: Can't implement 'double?' because it's nullable.
+// Try removing the question mark.
 // extension type E7(Null it) implements double? {} // Error.
 //                                       ^
 //

pkg/front_end/testcases/extension_types/issue53840.dart.weak.transformed.expect

@@ -19,7 +19,8 @@ library;
 // extension type E6(Null it) implements dynamic {} // Error.
 //                                       ^
 //
-// pkg/front_end/testcases/extension_types/issue53840.dart:22:39: Error: Can't implement 'double?' because it's marked with '?'.
+// pkg/front_end/testcases/extension_types/issue53840.dart:22:39: Error: Can't implement 'double?' because it's nullable.
+// Try removing the question mark.
 // extension type E7(Null it) implements double? {} // Error.
 //                                       ^
 //

pkg/front_end/testcases/general/issue54553.dart.strong.expect

@@ -2,7 +2,8 @@ library;
 //
 // Problems in library:
 //
-// pkg/front_end/testcases/general/issue54553.dart:8:7: Error: Can't implement 'A?' because it's marked with '?'.
+// pkg/front_end/testcases/general/issue54553.dart:8:7: Error: Can't implement 'A?' because it's nullable.
+// Try removing the question mark.
 // class B implements A? {} // Error.
 //       ^
 //
@@ -13,7 +14,8 @@ library;
 // typedef F<T> = T;
 //           ^
 //
-// pkg/front_end/testcases/general/issue54553.dart:5:46: Error: Can't implement 'int?' because it's marked with '?'.
+// pkg/front_end/testcases/general/issue54553.dart:5:46: Error: Can't implement 'int?' because it's nullable.
+// Try removing the question mark.
 // extension type EINullable(int? _) implements int? {} // Error.
 //                                              ^
 //

pkg/front_end/testcases/general/issue54553.dart.strong.transformed.expect

@@ -2,7 +2,8 @@ library;
 //
 // Problems in library:
 //
-// pkg/front_end/testcases/general/issue54553.dart:8:7: Error: Can't implement 'A?' because it's marked with '?'.
+// pkg/front_end/testcases/general/issue54553.dart:8:7: Error: Can't implement 'A?' because it's nullable.
+// Try removing the question mark.
 // class B implements A? {} // Error.
 //       ^
 //
@@ -13,7 +14,8 @@ library;
 // typedef F<T> = T;
 //           ^
 //
-// pkg/front_end/testcases/general/issue54553.dart:5:46: Error: Can't implement 'int?' because it's marked with '?'.
+// pkg/front_end/testcases/general/issue54553.dart:5:46: Error: Can't implement 'int?' because it's nullable.
+// Try removing the question mark.
 // extension type EINullable(int? _) implements int? {} // Error.
 //                                              ^
 //

pkg/front_end/testcases/general/issue54553.dart.weak.expect

@@ -2,7 +2,8 @@ library;
 //
 // Problems in library:
 //
-// pkg/front_end/testcases/general/issue54553.dart:8:7: Error: Can't implement 'A?' because it's marked with '?'.
+// pkg/front_end/testcases/general/issue54553.dart:8:7: Error: Can't implement 'A?' because it's nullable.
+// Try removing the question mark.
 // class B implements A? {} // Error.
 //       ^
 //
@@ -13,7 +14,8 @@ library;
 // typedef F<T> = T;
 //           ^
 //
-// pkg/front_end/testcases/general/issue54553.dart:5:46: Error: Can't implement 'int?' because it's marked with '?'.
+// pkg/front_end/testcases/general/issue54553.dart:5:46: Error: Can't implement 'int?' because it's nullable.
+// Try removing the question mark.
 // extension type EINullable(int? _) implements int? {} // Error.
 //                                              ^
 //

pkg/front_end/testcases/general/issue54553.dart.weak.modular.expect

@@ -2,7 +2,8 @@ library;
 //
 // Problems in library:
 //
-// pkg/front_end/testcases/general/issue54553.dart:8:7: Error: Can't implement 'A?' because it's marked with '?'.
+// pkg/front_end/testcases/general/issue54553.dart:8:7: Error: Can't implement 'A?' because it's nullable.
+// Try removing the question mark.
 // class B implements A? {} // Error.
 //       ^
 //
@@ -13,7 +14,8 @@ library;
 // typedef F<T> = T;
 //           ^
 //
-// pkg/front_end/testcases/general/issue54553.dart:5:46: Error: Can't implement 'int?' because it's marked with '?'.
+// pkg/front_end/testcases/general/issue54553.dart:5:46: Error: Can't implement 'int?' because it's nullable.
+// Try removing the question mark.
 // extension type EINullable(int? _) implements int? {} // Error.
 //                                              ^
 //

pkg/front_end/testcases/general/issue54553.dart.weak.outline.expect

@@ -2,7 +2,8 @@ library;
 //
 // Problems in library:
 //
-// pkg/front_end/testcases/general/issue54553.dart:8:7: Error: Can't implement 'A?' because it's marked with '?'.
+// pkg/front_end/testcases/general/issue54553.dart:8:7: Error: Can't implement 'A?' because it's nullable.
+// Try removing the question mark.
 // class B implements A? {} // Error.
 //       ^
 //
@@ -13,7 +14,8 @@ library;
 // typedef F<T> = T;
 //           ^
 //
-// pkg/front_end/testcases/general/issue54553.dart:5:46: Error: Can't implement 'int?' because it's marked with '?'.
+// pkg/front_end/testcases/general/issue54553.dart:5:46: Error: Can't implement 'int?' because it's nullable.
+// Try removing the question mark.
 // extension type EINullable(int? _) implements int? {} // Error.
 //                                              ^
 //

pkg/front_end/testcases/general/issue54553.dart.weak.transformed.expect

@@ -2,7 +2,8 @@ library;
 //
 // Problems in library:
 //
-// pkg/front_end/testcases/general/issue54553.dart:8:7: Error: Can't implement 'A?' because it's marked with '?'.
+// pkg/front_end/testcases/general/issue54553.dart:8:7: Error: Can't implement 'A?' because it's nullable.
+// Try removing the question mark.
 // class B implements A? {} // Error.
 //       ^
 //
@@ -13,7 +14,8 @@ library;
 // typedef F<T> = T;
 //           ^
 //
-// pkg/front_end/testcases/general/issue54553.dart:5:46: Error: Can't implement 'int?' because it's marked with '?'.
+// pkg/front_end/testcases/general/issue54553.dart:5:46: Error: Can't implement 'int?' because it's nullable.
+// Try removing the question mark.
 // extension type EINullable(int? _) implements int? {} // Error.
 //                                              ^
 //

pkg/front_end/testcases/nnbd/forbidden_supers.dart.strong.expect

@@ -2,59 +2,73 @@ library;
 //
 // Problems in library:
 //
-// pkg/front_end/testcases/nnbd/forbidden_supers.dart:12:7: Error: Can't mix 'Aoo?' in because it's marked with '?'.
+// pkg/front_end/testcases/nnbd/forbidden_supers.dart:12:7: Error: Can't mix 'Aoo?' in because it's nullable.
+// Try removing the question mark.
 // class Coo extends Boo with Aoo? {}
 //       ^^^
 //
-// pkg/front_end/testcases/nnbd/forbidden_supers.dart:14:7: Error: Can't extend 'Aoo?' because it's marked with '?'.
+// pkg/front_end/testcases/nnbd/forbidden_supers.dart:14:7: Error: Can't extend 'Aoo?' because it's nullable.
+// Try removing the question mark.
 // class Doo extends Aoo? {}
 //       ^^^
 //
-// pkg/front_end/testcases/nnbd/forbidden_supers.dart:16:7: Error: Can't implement 'Boo?' because it's marked with '?'.
+// pkg/front_end/testcases/nnbd/forbidden_supers.dart:16:7: Error: Can't implement 'Boo?' because it's nullable.
+// Try removing the question mark.
 // class Eoo implements Boo? {}
 //       ^^^
 //
-// pkg/front_end/testcases/nnbd/forbidden_supers.dart:18:7: Error: Can't extend 'Boo?' because it's marked with '?'.
+// pkg/front_end/testcases/nnbd/forbidden_supers.dart:18:7: Error: Can't extend 'Boo?' because it's nullable.
+// Try removing the question mark.
 // class Foo extends Boo? with Aoo {}
 //       ^^^
 //
-// pkg/front_end/testcases/nnbd/forbidden_supers.dart:20:7: Error: Can't extend 'Boo?' because it's marked with '?'.
+// pkg/front_end/testcases/nnbd/forbidden_supers.dart:20:7: Error: Can't extend 'Boo?' because it's nullable.
+// Try removing the question mark.
 // class Goo = Boo? with Aoo?;
 //       ^^^
 //
-// pkg/front_end/testcases/nnbd/forbidden_supers.dart:20:7: Error: Can't mix 'Aoo?' in because it's marked with '?'.
+// pkg/front_end/testcases/nnbd/forbidden_supers.dart:20:7: Error: Can't mix 'Aoo?' in because it's nullable.
+// Try removing the question mark.
 // class Goo = Boo? with Aoo?;
 //       ^^^
 //
-// pkg/front_end/testcases/nnbd/forbidden_supers.dart:22:7: Error: Can't implement 'Boo?' because it's marked with '?'.
+// pkg/front_end/testcases/nnbd/forbidden_supers.dart:22:7: Error: Can't implement 'Boo?' because it's nullable.
+// Try removing the question mark.
 // class Hoo extends Object with Aoo implements Boo? {}
 //       ^^^
 //
-// pkg/front_end/testcases/nnbd/forbidden_supers.dart:24:7: Error: Can't implement 'Boo?' because it's marked with '?'.
+// pkg/front_end/testcases/nnbd/forbidden_supers.dart:24:7: Error: Can't implement 'Boo?' because it's nullable.
+// Try removing the question mark.
 // class Ioo = Object with Aoo implements Boo?;
 //       ^^^
 //
-// pkg/front_end/testcases/nnbd/forbidden_supers.dart:32:7: Error: Can't extend 'Aoo?' because it's marked with '?'.
+// pkg/front_end/testcases/nnbd/forbidden_supers.dart:32:7: Error: Can't extend 'Aoo?' because it's nullable.
+// Try removing the question mark.
 // mixin Moo1 on Aoo? implements Boo? {}
 //       ^^^^
 //
-// pkg/front_end/testcases/nnbd/forbidden_supers.dart:32:7: Error: Can't implement 'Boo?' because it's marked with '?'.
+// pkg/front_end/testcases/nnbd/forbidden_supers.dart:32:7: Error: Can't implement 'Boo?' because it's nullable.
+// Try removing the question mark.
 // mixin Moo1 on Aoo? implements Boo? {}
 //       ^^^^
 //
-// pkg/front_end/testcases/nnbd/forbidden_supers.dart:34:7: Error: Can't extend 'Aoo?' because it's marked with '?'.
+// pkg/front_end/testcases/nnbd/forbidden_supers.dart:34:7: Error: Can't extend 'Aoo?' because it's nullable.
+// Try removing the question mark.
 // mixin Moo2 on Aoo?, Boo? {}
 //       ^^^^
 //
-// pkg/front_end/testcases/nnbd/forbidden_supers.dart:34:7: Error: Can't extend 'Boo?' because it's marked with '?'.
+// pkg/front_end/testcases/nnbd/forbidden_supers.dart:34:7: Error: Can't extend 'Boo?' because it's nullable.
+// Try removing the question mark.
 // mixin Moo2 on Aoo?, Boo? {}
 //       ^^^^
 //
-// pkg/front_end/testcases/nnbd/forbidden_supers.dart:36:7: Error: Can't implement 'Aoo?' because it's marked with '?'.
+// pkg/front_end/testcases/nnbd/forbidden_supers.dart:36:7: Error: Can't implement 'Aoo?' because it's nullable.
+// Try removing the question mark.
 // mixin Moo3 implements Aoo?, Boo? {}
 //       ^^^^
 //
-// pkg/front_end/testcases/nnbd/forbidden_supers.dart:36:7: Error: Can't implement 'Boo?' because it's marked with '?'.
+// pkg/front_end/testcases/nnbd/forbidden_supers.dart:36:7: Error: Can't implement 'Boo?' because it's nullable.
+// Try removing the question mark.
 // mixin Moo3 implements Aoo?, Boo? {}
 //       ^^^^
 //

pkg/front_end/testcases/nnbd/forbidden_supers.dart.strong.transformed.expect

@@ -2,59 +2,73 @@ library;
 //
 // Problems in library:
 //
-// pkg/front_end/testcases/nnbd/forbidden_supers.dart:12:7: Error: Can't mix 'Aoo?' in because it's marked with '?'.
+// pkg/front_end/testcases/nnbd/forbidden_supers.dart:12:7: Error: Can't mix 'Aoo?' in because it's nullable.
+// Try removing the question mark.
 // class Coo extends Boo with Aoo? {}
 //       ^^^
 //
-// pkg/front_end/testcases/nnbd/forbidden_supers.dart:14:7: Error: Can't extend 'Aoo?' because it's marked with '?'.
+// pkg/front_end/testcases/nnbd/forbidden_supers.dart:14:7: Error: Can't extend 'Aoo?' because it's nullable.
+// Try removing the question mark.
 // class Doo extends Aoo? {}
 //       ^^^
 //
-// pkg/front_end/testcases/nnbd/forbidden_supers.dart:16:7: Error: Can't implement 'Boo?' because it's marked with '?'.
+// pkg/front_end/testcases/nnbd/forbidden_supers.dart:16:7: Error: Can't implement 'Boo?' because it's nullable.
+// Try removing the question mark.
 // class Eoo implements Boo? {}
 //       ^^^
 //
-// pkg/front_end/testcases/nnbd/forbidden_supers.dart:18:7: Error: Can't extend 'Boo?' because it's marked with '?'.
+// pkg/front_end/testcases/nnbd/forbidden_supers.dart:18:7: Error: Can't extend 'Boo?' because it's nullable.
+// Try removing the question mark.
 // class Foo extends Boo? with Aoo {}
 //       ^^^
 //
-// pkg/front_end/testcases/nnbd/forbidden_supers.dart:20:7: Error: Can't extend 'Boo?' because it's marked with '?'.
+// pkg/front_end/testcases/nnbd/forbidden_supers.dart:20:7: Error: Can't extend 'Boo?' because it's nullable.
+// Try removing the question mark.
 // class Goo = Boo? with Aoo?;
 //       ^^^
 //
-// pkg/front_end/testcases/nnbd/forbidden_supers.dart:20:7: Error: Can't mix 'Aoo?' in because it's marked with '?'.
+// pkg/front_end/testcases/nnbd/forbidden_supers.dart:20:7: Error: Can't mix 'Aoo?' in because it's nullable.
+// Try removing the question mark.
 // class Goo = Boo? with Aoo?;
 //       ^^^
 //
-// pkg/front_end/testcases/nnbd/forbidden_supers.dart:22:7: Error: Can't implement 'Boo?' because it's marked with '?'.
+// pkg/front_end/testcases/nnbd/forbidden_supers.dart:22:7: Error: Can't implement 'Boo?' because it's nullable.
+// Try removing the question mark.
 // class Hoo extends Object with Aoo implements Boo? {}
 //       ^^^
 //
-// pkg/front_end/testcases/nnbd/forbidden_supers.dart:24:7: Error: Can't implement 'Boo?' because it's marked with '?'.
+// pkg/front_end/testcases/nnbd/forbidden_supers.dart:24:7: Error: Can't implement 'Boo?' because it's nullable.
+// Try removing the question mark.
 // class Ioo = Object with Aoo implements Boo?;
 //       ^^^
 //
-// pkg/front_end/testcases/nnbd/forbidden_supers.dart:32:7: Error: Can't extend 'Aoo?' because it's marked with '?'.
+// pkg/front_end/testcases/nnbd/forbidden_supers.dart:32:7: Error: Can't extend 'Aoo?' because it's nullable.
+// Try removing the question mark.
 // mixin Moo1 on Aoo? implements Boo? {}
 //       ^^^^
 //
-// pkg/front_end/testcases/nnbd/forbidden_supers.dart:32:7: Error: Can't implement 'Boo?' because it's marked with '?'.
+// pkg/front_end/testcases/nnbd/forbidden_supers.dart:32:7: Error: Can't implement 'Boo?' because it's nullable.
+// Try removing the question mark.
 // mixin Moo1 on Aoo? implements Boo? {}
 //       ^^^^
 //
-// pkg/front_end/testcases/nnbd/forbidden_supers.dart:34:7: Error: Can't extend 'Aoo?' because it's marked with '?'.
+// pkg/front_end/testcases/nnbd/forbidden_supers.dart:34:7: Error: Can't extend 'Aoo?' because it's nullable.
+// Try removing the question mark.
 // mixin Moo2 on Aoo?, Boo? {}
 //       ^^^^
 //
-// pkg/front_end/testcases/nnbd/forbidden_supers.dart:34:7: Error: Can't extend 'Boo?' because it's marked with '?'.
+// pkg/front_end/testcases/nnbd/forbidden_supers.dart:34:7: Error: Can't extend 'Boo?' because it's nullable.
+// Try removing the question mark.
 // mixin Moo2 on Aoo?, Boo? {}
 //       ^^^^
 //
-// pkg/front_end/testcases/nnbd/forbidden_supers.dart:36:7: Error: Can't implement 'Aoo?' because it's marked with '?'.
+// pkg/front_end/testcases/nnbd/forbidden_supers.dart:36:7: Error: Can't implement 'Aoo?' because it's nullable.
+// Try removing the question mark.
 // mixin Moo3 implements Aoo?, Boo? {}
 //       ^^^^
 //
-// pkg/front_end/testcases/nnbd/forbidden_supers.dart:36:7: Error: Can't implement 'Boo?' because it's marked with '?'.
+// pkg/front_end/testcases/nnbd/forbidden_supers.dart:36:7: Error: Can't implement 'Boo?' because it's nullable.
+// Try removing the question mark.
 // mixin Moo3 implements Aoo?, Boo? {}
 //       ^^^^
 //

pkg/front_end/testcases/nnbd/forbidden_supers.dart.weak.expect

@@ -2,59 +2,73 @@ library;
 //
 // Problems in library:
 //
-// pkg/front_end/testcases/nnbd/forbidden_supers.dart:12:7: Error: Can't mix 'Aoo?' in because it's marked with '?'.
+// pkg/front_end/testcases/nnbd/forbidden_supers.dart:12:7: Error: Can't mix 'Aoo?' in because it's nullable.
+// Try removing the question mark.
 // class Coo extends Boo with Aoo? {}
 //       ^^^
 //
-// pkg/front_end/testcases/nnbd/forbidden_supers.dart:14:7: Error: Can't extend 'Aoo?' because it's marked with '?'.
+// pkg/front_end/testcases/nnbd/forbidden_supers.dart:14:7: Error: Can't extend 'Aoo?' because it's nullable.
+// Try removing the question mark.
 // class Doo extends Aoo? {}
 //       ^^^
 //
-// pkg/front_end/testcases/nnbd/forbidden_supers.dart:16:7: Error: Can't implement 'Boo?' because it's marked with '?'.
+// pkg/front_end/testcases/nnbd/forbidden_supers.dart:16:7: Error: Can't implement 'Boo?' because it's nullable.
+// Try removing the question mark.
 // class Eoo implements Boo? {}
 //       ^^^
 //
-// pkg/front_end/testcases/nnbd/forbidden_supers.dart:18:7: Error: Can't extend 'Boo?' because it's marked with '?'.
+// pkg/front_end/testcases/nnbd/forbidden_supers.dart:18:7: Error: Can't extend 'Boo?' because it's nullable.
+// Try removing the question mark.
 // class Foo extends Boo? with Aoo {}
 //       ^^^
 //
-// pkg/front_end/testcases/nnbd/forbidden_supers.dart:20:7: Error: Can't extend 'Boo?' because it's marked with '?'.
+// pkg/front_end/testcases/nnbd/forbidden_supers.dart:20:7: Error: Can't extend 'Boo?' because it's nullable.
+// Try removing the question mark.
 // class Goo = Boo? with Aoo?;
 //       ^^^
 //
-// pkg/front_end/testcases/nnbd/forbidden_supers.dart:20:7: Error: Can't mix 'Aoo?' in because it's marked with '?'.
+// pkg/front_end/testcases/nnbd/forbidden_supers.dart:20:7: Error: Can't mix 'Aoo?' in because it's nullable.
+// Try removing the question mark.
 // class Goo = Boo? with Aoo?;
 //       ^^^
 //
-// pkg/front_end/testcases/nnbd/forbidden_supers.dart:22:7: Error: Can't implement 'Boo?' because it's marked with '?'.
+// pkg/front_end/testcases/nnbd/forbidden_supers.dart:22:7: Error: Can't implement 'Boo?' because it's nullable.
+// Try removing the question mark.
 // class Hoo extends Object with Aoo implements Boo? {}
 //       ^^^
 //
-// pkg/front_end/testcases/nnbd/forbidden_supers.dart:24:7: Error: Can't implement 'Boo?' because it's marked with '?'.
+// pkg/front_end/testcases/nnbd/forbidden_supers.dart:24:7: Error: Can't implement 'Boo?' because it's nullable.
+// Try removing the question mark.
 // class Ioo = Object with Aoo implements Boo?;
 //       ^^^
 //
-// pkg/front_end/testcases/nnbd/forbidden_supers.dart:32:7: Error: Can't extend 'Aoo?' because it's marked with '?'.
+// pkg/front_end/testcases/nnbd/forbidden_supers.dart:32:7: Error: Can't extend 'Aoo?' because it's nullable.
+// Try removing the question mark.
 // mixin Moo1 on Aoo? implements Boo? {}
 //       ^^^^
 //
-// pkg/front_end/testcases/nnbd/forbidden_supers.dart:32:7: Error: Can't implement 'Boo?' because it's marked with '?'.
+// pkg/front_end/testcases/nnbd/forbidden_supers.dart:32:7: Error: Can't implement 'Boo?' because it's nullable.
+// Try removing the question mark.
 // mixin Moo1 on Aoo? implements Boo? {}
 //       ^^^^
 //
-// pkg/front_end/testcases/nnbd/forbidden_supers.dart:34:7: Error: Can't extend 'Aoo?' because it's marked with '?'.
+// pkg/front_end/testcases/nnbd/forbidden_supers.dart:34:7: Error: Can't extend 'Aoo?' because it's nullable.
+// Try removing the question mark.
 // mixin Moo2 on Aoo?, Boo? {}
 //       ^^^^
 //
-// pkg/front_end/testcases/nnbd/forbidden_supers.dart:34:7: Error: Can't extend 'Boo?' because it's marked with '?'.
+// pkg/front_end/testcases/nnbd/forbidden_supers.dart:34:7: Error: Can't extend 'Boo?' because it's nullable.
+// Try removing the question mark.
 // mixin Moo2 on Aoo?, Boo? {}
 //       ^^^^
 //
-// pkg/front_end/testcases/nnbd/forbidden_supers.dart:36:7: Error: Can't implement 'Aoo?' because it's marked with '?'.
+// pkg/front_end/testcases/nnbd/forbidden_supers.dart:36:7: Error: Can't implement 'Aoo?' because it's nullable.
+// Try removing the question mark.
 // mixin Moo3 implements Aoo?, Boo? {}
 //       ^^^^
 //
-// pkg/front_end/testcases/nnbd/forbidden_supers.dart:36:7: Error: Can't implement 'Boo?' because it's marked with '?'.
+// pkg/front_end/testcases/nnbd/forbidden_supers.dart:36:7: Error: Can't implement 'Boo?' because it's nullable.
+// Try removing the question mark.
 // mixin Moo3 implements Aoo?, Boo? {}
 //       ^^^^
 //

pkg/front_end/testcases/nnbd/forbidden_supers.dart.weak.modular.expect

@@ -2,59 +2,73 @@ library;
 //
 // Problems in library:
 //
-// pkg/front_end/testcases/nnbd/forbidden_supers.dart:12:7: Error: Can't mix 'Aoo?' in because it's marked with '?'.
+// pkg/front_end/testcases/nnbd/forbidden_supers.dart:12:7: Error: Can't mix 'Aoo?' in because it's nullable.
+// Try removing the question mark.
 // class Coo extends Boo with Aoo? {}
 //       ^^^
 //
-// pkg/front_end/testcases/nnbd/forbidden_supers.dart:14:7: Error: Can't extend 'Aoo?' because it's marked with '?'.
+// pkg/front_end/testcases/nnbd/forbidden_supers.dart:14:7: Error: Can't extend 'Aoo?' because it's nullable.
+// Try removing the question mark.
 // class Doo extends Aoo? {}
 //       ^^^
 //
-// pkg/front_end/testcases/nnbd/forbidden_supers.dart:16:7: Error: Can't implement 'Boo?' because it's marked with '?'.
+// pkg/front_end/testcases/nnbd/forbidden_supers.dart:16:7: Error: Can't implement 'Boo?' because it's nullable.
+// Try removing the question mark.
 // class Eoo implements Boo? {}
 //       ^^^
 //
-// pkg/front_end/testcases/nnbd/forbidden_supers.dart:18:7: Error: Can't extend 'Boo?' because it's marked with '?'.
+// pkg/front_end/testcases/nnbd/forbidden_supers.dart:18:7: Error: Can't extend 'Boo?' because it's nullable.
+// Try removing the question mark.
 // class Foo extends Boo? with Aoo {}
 //       ^^^
 //
-// pkg/front_end/testcases/nnbd/forbidden_supers.dart:20:7: Error: Can't extend 'Boo?' because it's marked with '?'.
+// pkg/front_end/testcases/nnbd/forbidden_supers.dart:20:7: Error: Can't extend 'Boo?' because it's nullable.
+// Try removing the question mark.
 // class Goo = Boo? with Aoo?;
 //       ^^^
 //
-// pkg/front_end/testcases/nnbd/forbidden_supers.dart:20:7: Error: Can't mix 'Aoo?' in because it's marked with '?'.
+// pkg/front_end/testcases/nnbd/forbidden_supers.dart:20:7: Error: Can't mix 'Aoo?' in because it's nullable.
+// Try removing the question mark.
 // class Goo = Boo? with Aoo?;
 //       ^^^
 //
-// pkg/front_end/testcases/nnbd/forbidden_supers.dart:22:7: Error: Can't implement 'Boo?' because it's marked with '?'.
+// pkg/front_end/testcases/nnbd/forbidden_supers.dart:22:7: Error: Can't implement 'Boo?' because it's nullable.
+// Try removing the question mark.
 // class Hoo extends Object with Aoo implements Boo? {}
 //       ^^^
 //
-// pkg/front_end/testcases/nnbd/forbidden_supers.dart:24:7: Error: Can't implement 'Boo?' because it's marked with '?'.
+// pkg/front_end/testcases/nnbd/forbidden_supers.dart:24:7: Error: Can't implement 'Boo?' because it's nullable.
+// Try removing the question mark.
 // class Ioo = Object with Aoo implements Boo?;
 //       ^^^
 //
-// pkg/front_end/testcases/nnbd/forbidden_supers.dart:32:7: Error: Can't extend 'Aoo?' because it's marked with '?'.
+// pkg/front_end/testcases/nnbd/forbidden_supers.dart:32:7: Error: Can't extend 'Aoo?' because it's nullable.
+// Try removing the question mark.
 // mixin Moo1 on Aoo? implements Boo? {}
 //       ^^^^
 //
-// pkg/front_end/testcases/nnbd/forbidden_supers.dart:32:7: Error: Can't implement 'Boo?' because it's marked with '?'.
+// pkg/front_end/testcases/nnbd/forbidden_supers.dart:32:7: Error: Can't implement 'Boo?' because it's nullable.
+// Try removing the question mark.
 // mixin Moo1 on Aoo? implements Boo? {}
 //       ^^^^
 //
-// pkg/front_end/testcases/nnbd/forbidden_supers.dart:34:7: Error: Can't extend 'Aoo?' because it's marked with '?'.
+// pkg/front_end/testcases/nnbd/forbidden_supers.dart:34:7: Error: Can't extend 'Aoo?' because it's nullable.
+// Try removing the question mark.
 // mixin Moo2 on Aoo?, Boo? {}
 //       ^^^^
 //
-// pkg/front_end/testcases/nnbd/forbidden_supers.dart:34:7: Error: Can't extend 'Boo?' because it's marked with '?'.
+// pkg/front_end/testcases/nnbd/forbidden_supers.dart:34:7: Error: Can't extend 'Boo?' because it's nullable.
+// Try removing the question mark.
 // mixin Moo2 on Aoo?, Boo? {}
 //       ^^^^
 //
-// pkg/front_end/testcases/nnbd/forbidden_supers.dart:36:7: Error: Can't implement 'Aoo?' because it's marked with '?'.
+// pkg/front_end/testcases/nnbd/forbidden_supers.dart:36:7: Error: Can't implement 'Aoo?' because it's nullable.
+// Try removing the question mark.
 // mixin Moo3 implements Aoo?, Boo? {}
 //       ^^^^
 //
-// pkg/front_end/testcases/nnbd/forbidden_supers.dart:36:7: Error: Can't implement 'Boo?' because it's marked with '?'.
+// pkg/front_end/testcases/nnbd/forbidden_supers.dart:36:7: Error: Can't implement 'Boo?' because it's nullable.
+// Try removing the question mark.
 // mixin Moo3 implements Aoo?, Boo? {}
 //       ^^^^
 //

pkg/front_end/testcases/nnbd/forbidden_supers.dart.weak.outline.expect

@@ -2,59 +2,73 @@ library;
 //
 // Problems in library:
 //
-// pkg/front_end/testcases/nnbd/forbidden_supers.dart:12:7: Error: Can't mix 'Aoo?' in because it's marked with '?'.
+// pkg/front_end/testcases/nnbd/forbidden_supers.dart:12:7: Error: Can't mix 'Aoo?' in because it's nullable.
+// Try removing the question mark.
 // class Coo extends Boo with Aoo? {}
 //       ^^^
 //
-// pkg/front_end/testcases/nnbd/forbidden_supers.dart:14:7: Error: Can't extend 'Aoo?' because it's marked with '?'.
+// pkg/front_end/testcases/nnbd/forbidden_supers.dart:14:7: Error: Can't extend 'Aoo?' because it's nullable.
+// Try removing the question mark.
 // class Doo extends Aoo? {}
 //       ^^^
 //
-// pkg/front_end/testcases/nnbd/forbidden_supers.dart:16:7: Error: Can't implement 'Boo?' because it's marked with '?'.
+// pkg/front_end/testcases/nnbd/forbidden_supers.dart:16:7: Error: Can't implement 'Boo?' because it's nullable.
+// Try removing the question mark.
 // class Eoo implements Boo? {}
 //       ^^^
 //
-// pkg/front_end/testcases/nnbd/forbidden_supers.dart:18:7: Error: Can't extend 'Boo?' because it's marked with '?'.
+// pkg/front_end/testcases/nnbd/forbidden_supers.dart:18:7: Error: Can't extend 'Boo?' because it's nullable.
+// Try removing the question mark.
 // class Foo extends Boo? with Aoo {}
 //       ^^^
 //
-// pkg/front_end/testcases/nnbd/forbidden_supers.dart:20:7: Error: Can't extend 'Boo?' because it's marked with '?'.
+// pkg/front_end/testcases/nnbd/forbidden_supers.dart:20:7: Error: Can't extend 'Boo?' because it's nullable.
+// Try removing the question mark.
 // class Goo = Boo? with Aoo?;
 //       ^^^
 //
-// pkg/front_end/testcases/nnbd/forbidden_supers.dart:20:7: Error: Can't mix 'Aoo?' in because it's marked with '?'.
+// pkg/front_end/testcases/nnbd/forbidden_supers.dart:20:7: Error: Can't mix 'Aoo?' in because it's nullable.
+// Try removing the question mark.
 // class Goo = Boo? with Aoo?;
 //       ^^^
 //
-// pkg/front_end/testcases/nnbd/forbidden_supers.dart:22:7: Error: Can't implement 'Boo?' because it's marked with '?'.
+// pkg/front_end/testcases/nnbd/forbidden_supers.dart:22:7: Error: Can't implement 'Boo?' because it's nullable.
+// Try removing the question mark.
 // class Hoo extends Object with Aoo implements Boo? {}
 //       ^^^
 //
-// pkg/front_end/testcases/nnbd/forbidden_supers.dart:24:7: Error: Can't implement 'Boo?' because it's marked with '?'.
+// pkg/front_end/testcases/nnbd/forbidden_supers.dart:24:7: Error: Can't implement 'Boo?' because it's nullable.
+// Try removing the question mark.
 // class Ioo = Object with Aoo implements Boo?;
 //       ^^^
 //
-// pkg/front_end/testcases/nnbd/forbidden_supers.dart:32:7: Error: Can't extend 'Aoo?' because it's marked with '?'.
+// pkg/front_end/testcases/nnbd/forbidden_supers.dart:32:7: Error: Can't extend 'Aoo?' because it's nullable.
+// Try removing the question mark.
 // mixin Moo1 on Aoo? implements Boo? {}
 //       ^^^^
 //
-// pkg/front_end/testcases/nnbd/forbidden_supers.dart:32:7: Error: Can't implement 'Boo?' because it's marked with '?'.
+// pkg/front_end/testcases/nnbd/forbidden_supers.dart:32:7: Error: Can't implement 'Boo?' because it's nullable.
+// Try removing the question mark.
 // mixin Moo1 on Aoo? implements Boo? {}
 //       ^^^^
 //
-// pkg/front_end/testcases/nnbd/forbidden_supers.dart:34:7: Error: Can't extend 'Aoo?' because it's marked with '?'.
+// pkg/front_end/testcases/nnbd/forbidden_supers.dart:34:7: Error: Can't extend 'Aoo?' because it's nullable.
+// Try removing the question mark.
 // mixin Moo2 on Aoo?, Boo? {}
 //       ^^^^
 //
-// pkg/front_end/testcases/nnbd/forbidden_supers.dart:34:7: Error: Can't extend 'Boo?' because it's marked with '?'.
+// pkg/front_end/testcases/nnbd/forbidden_supers.dart:34:7: Error: Can't extend 'Boo?' because it's nullable.
+// Try removing the question mark.
 // mixin Moo2 on Aoo?, Boo? {}
 //       ^^^^
 //
-// pkg/front_end/testcases/nnbd/forbidden_supers.dart:36:7: Error: Can't implement 'Aoo?' because it's marked with '?'.
+// pkg/front_end/testcases/nnbd/forbidden_supers.dart:36:7: Error: Can't implement 'Aoo?' because it's nullable.
+// Try removing the question mark.
 // mixin Moo3 implements Aoo?, Boo? {}
 //       ^^^^
 //
-// pkg/front_end/testcases/nnbd/forbidden_supers.dart:36:7: Error: Can't implement 'Boo?' because it's marked with '?'.
+// pkg/front_end/testcases/nnbd/forbidden_supers.dart:36:7: Error: Can't implement 'Boo?' because it's nullable.
+// Try removing the question mark.
 // mixin Moo3 implements Aoo?, Boo? {}
 //       ^^^^
 //

pkg/front_end/testcases/nnbd/forbidden_supers.dart.weak.transformed.expect

@@ -2,59 +2,73 @@ library;
 //
 // Problems in library:
 //
-// pkg/front_end/testcases/nnbd/forbidden_supers.dart:12:7: Error: Can't mix 'Aoo?' in because it's marked with '?'.
+// pkg/front_end/testcases/nnbd/forbidden_supers.dart:12:7: Error: Can't mix 'Aoo?' in because it's nullable.
+// Try removing the question mark.
 // class Coo extends Boo with Aoo? {}
 //       ^^^
 //
-// pkg/front_end/testcases/nnbd/forbidden_supers.dart:14:7: Error: Can't extend 'Aoo?' because it's marked with '?'.
+// pkg/front_end/testcases/nnbd/forbidden_supers.dart:14:7: Error: Can't extend 'Aoo?' because it's nullable.
+// Try removing the question mark.
 // class Doo extends Aoo? {}
 //       ^^^
 //
-// pkg/front_end/testcases/nnbd/forbidden_supers.dart:16:7: Error: Can't implement 'Boo?' because it's marked with '?'.
+// pkg/front_end/testcases/nnbd/forbidden_supers.dart:16:7: Error: Can't implement 'Boo?' because it's nullable.
+// Try removing the question mark.
 // class Eoo implements Boo? {}
 //       ^^^
 //
-// pkg/front_end/testcases/nnbd/forbidden_supers.dart:18:7: Error: Can't extend 'Boo?' because it's marked with '?'.
+// pkg/front_end/testcases/nnbd/forbidden_supers.dart:18:7: Error: Can't extend 'Boo?' because it's nullable.
+// Try removing the question mark.
 // class Foo extends Boo? with Aoo {}
 //       ^^^
 //
-// pkg/front_end/testcases/nnbd/forbidden_supers.dart:20:7: Error: Can't extend 'Boo?' because it's marked with '?'.
+// pkg/front_end/testcases/nnbd/forbidden_supers.dart:20:7: Error: Can't extend 'Boo?' because it's nullable.
+// Try removing the question mark.
 // class Goo = Boo? with Aoo?;
 //       ^^^
 //
-// pkg/front_end/testcases/nnbd/forbidden_supers.dart:20:7: Error: Can't mix 'Aoo?' in because it's marked with '?'.
+// pkg/front_end/testcases/nnbd/forbidden_supers.dart:20:7: Error: Can't mix 'Aoo?' in because it's nullable.
+// Try removing the question mark.
 // class Goo = Boo? with Aoo?;
 //       ^^^
 //
-// pkg/front_end/testcases/nnbd/forbidden_supers.dart:22:7: Error: Can't implement 'Boo?' because it's marked with '?'.
+// pkg/front_end/testcases/nnbd/forbidden_supers.dart:22:7: Error: Can't implement 'Boo?' because it's nullable.
+// Try removing the question mark.
 // class Hoo extends Object with Aoo implements Boo? {}
 //       ^^^
 //
-// pkg/front_end/testcases/nnbd/forbidden_supers.dart:24:7: Error: Can't implement 'Boo?' because it's marked with '?'.
+// pkg/front_end/testcases/nnbd/forbidden_supers.dart:24:7: Error: Can't implement 'Boo?' because it's nullable.
+// Try removing the question mark.
 // class Ioo = Object with Aoo implements Boo?;
 //       ^^^
 //
-// pkg/front_end/testcases/nnbd/forbidden_supers.dart:32:7: Error: Can't extend 'Aoo?' because it's marked with '?'.
+// pkg/front_end/testcases/nnbd/forbidden_supers.dart:32:7: Error: Can't extend 'Aoo?' because it's nullable.
+// Try removing the question mark.
 // mixin Moo1 on Aoo? implements Boo? {}
 //       ^^^^
 //
-// pkg/front_end/testcases/nnbd/forbidden_supers.dart:32:7: Error: Can't implement 'Boo?' because it's marked with '?'.
+// pkg/front_end/testcases/nnbd/forbidden_supers.dart:32:7: Error: Can't implement 'Boo?' because it's nullable.
+// Try removing the question mark.
 // mixin Moo1 on Aoo? implements Boo? {}
 //       ^^^^
 //
-// pkg/front_end/testcases/nnbd/forbidden_supers.dart:34:7: Error: Can't extend 'Aoo?' because it's marked with '?'.
+// pkg/front_end/testcases/nnbd/forbidden_supers.dart:34:7: Error: Can't extend 'Aoo?' because it's nullable.
+// Try removing the question mark.
 // mixin Moo2 on Aoo?, Boo? {}
 //       ^^^^
 //
-// pkg/front_end/testcases/nnbd/forbidden_supers.dart:34:7: Error: Can't extend 'Boo?' because it's marked with '?'.
+// pkg/front_end/testcases/nnbd/forbidden_supers.dart:34:7: Error: Can't extend 'Boo?' because it's nullable.
+// Try removing the question mark.
 // mixin Moo2 on Aoo?, Boo? {}
 //       ^^^^
 //
-// pkg/front_end/testcases/nnbd/forbidden_supers.dart:36:7: Error: Can't implement 'Aoo?' because it's marked with '?'.
+// pkg/front_end/testcases/nnbd/forbidden_supers.dart:36:7: Error: Can't implement 'Aoo?' because it's nullable.
+// Try removing the question mark.
 // mixin Moo3 implements Aoo?, Boo? {}
 //       ^^^^
 //
-// pkg/front_end/testcases/nnbd/forbidden_supers.dart:36:7: Error: Can't implement 'Boo?' because it's marked with '?'.
+// pkg/front_end/testcases/nnbd/forbidden_supers.dart:36:7: Error: Can't implement 'Boo?' because it's nullable.
+// Try removing the question mark.
 // mixin Moo3 implements Aoo?, Boo? {}
 //       ^^^^
 //