[_fe_analyzer_shared] Add SharedNullTypeStructure class.

This is the shared base class for all representations of the type
`Null`. This allows the shared codebase to use `is` tests to tell when
a type is `Null`.

Change-Id: I98059b60c7eaab9c9f1e3f7addb7913dffc9cf9d
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/396380
Reviewed-by: Chloe Stefantsova <cstefantsova@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>

pkg/_fe_analyzer_shared/lib/src/type_inference/type_analyzer_operations.dart

@@ -208,9 +208,6 @@ abstract interface class TypeAnalyzerOperations<
   /// a subtype query, but the implementations can do that more efficiently.
   bool isNonNullableInternal(TypeStructure type);
 
-  /// Returns `true` if [type] is `Null`.
-  bool isNullInternal(TypeStructure type);
-
   /// Returns `true` if [type] is `Object` from `dart:core`. The method returns
   /// `false` for `Object?` and `Object*`.
   bool isObject(SharedTypeView<TypeStructure> type);
@@ -1665,8 +1662,7 @@ abstract class TypeConstraintGenerator<
 
     // If `P` is `Null`, then the match holds under no constraints:
     //  Only if `Q` is nullable.
-    if (pNullability == NullabilitySuffix.none &&
-        typeAnalyzerOperations.isNullInternal(p)) {
+    if (p is SharedNullTypeStructure) {
       return typeAnalyzerOperations.isNullableInternal(q);
     }
 

pkg/_fe_analyzer_shared/lib/src/types/shared_type.dart

@@ -66,6 +66,12 @@ abstract interface class SharedNamedTypeStructure<
   TypeStructure get type;
 }
 
+/// Common interface for data structures used by implementations to represent
+/// the type `Null`.
+abstract interface class SharedNullTypeStructure<
+        TypeStructure extends SharedTypeStructure<TypeStructure>>
+    implements SharedTypeStructure<TypeStructure> {}
+
 /// Common interface for data structures used by the implementations to
 /// represent a record type.
 abstract interface class SharedRecordTypeStructure<

pkg/_fe_analyzer_shared/test/mini_ast.dart

@@ -2990,9 +2990,6 @@ class MiniAstOperations
         unwrappedType.nullabilitySuffix == NullabilitySuffix.none;
   }
 
-  @override
-  bool isNullInternal(Type type) => type is NullType;
-
   @override
   bool isObject(SharedTypeView<Type> type) {
     Type unwrappedType = type.unwrapTypeView();

pkg/_fe_analyzer_shared/test/mini_types.dart

@@ -480,7 +480,8 @@ class NeverType extends _SpecialSimpleType {
 
 /// Representation of the type `Null` suitable for unit testing of code in the
 /// `_fe_analyzer_shared` package.
-class NullType extends _SpecialSimpleType {
+class NullType extends _SpecialSimpleType
+    implements SharedNullTypeStructure<Type> {
   static final instance = NullType._();
 
   NullType._()

pkg/analyzer/lib/src/dart/element/type.dart

@@ -1142,7 +1142,8 @@ class NeverTypeImpl extends TypeImpl implements NeverType {
 
 /// A concrete implementation of [DartType] representing the type `Null`, with
 /// no type parameters and no nullability suffix.
-class NullTypeImpl extends InterfaceTypeImpl {
+class NullTypeImpl extends InterfaceTypeImpl
+    implements SharedNullTypeStructure<DartType> {
   NullTypeImpl({required super.element, super.alias}) : super._null();
 
   @override

pkg/analyzer/lib/src/dart/resolver/flow_analysis_visitor.dart

@@ -585,9 +585,6 @@ class TypeSystemOperations
     return typeSystem.isNullable(type);
   }
 
-  @override
-  bool isNullInternal(DartType type) => type.isDartCoreNull;
-
   @override
   bool isObject(SharedTypeView<DartType> type) {
     return type.unwrapTypeView().isDartCoreObject &&

pkg/front_end/lib/src/type_inference/type_inference_engine.dart

@@ -569,9 +569,6 @@ class OperationsCfe
     return typeEnvironment.coreTypes.isBottom(type.unwrapTypeView());
   }
 
-  @override
-  bool isNullInternal(DartType type) => type is NullType;
-
   @override
   // Coverage-ignore(suite): Not run.
   bool isObject(SharedTypeView<DartType> type) {

pkg/kernel/lib/ast.dart

@@ -74,10 +74,11 @@ import 'package:_fe_analyzer_shared/src/type_inference/type_analyzer_operations.
 import 'package:_fe_analyzer_shared/src/types/shared_type.dart'
     show
         SharedDynamicTypeStructure,
-        SharedNamedFunctionParameterStructure,
         SharedFunctionTypeStructure,
         SharedInvalidTypeStructure,
+        SharedNamedFunctionParameterStructure,
         SharedNamedTypeStructure,
+        SharedNullTypeStructure,
         SharedRecordTypeStructure,
         SharedTypeParameterStructure,
         SharedTypeStructure,

pkg/kernel/lib/src/ast/types.dart

@@ -774,7 +774,7 @@ class NeverType extends DartType {
   }
 }
 
-class NullType extends DartType {
+class NullType extends DartType implements SharedNullTypeStructure<DartType> {
   @override
   final int hashCode = 415324;