[analyzer] Rework InterfaceTypeImpl constructor to call NullTypeImpl

Previously, the `InterfaceTypeImpl._` constructor threw an exception
if an interface type was being constructed for the type `Null`
(because the `NullTypeImpl` constructor should be used instead). This
was silly; as long as we're going to check, we might as well put the
check in the `InterfaceTypeImpl` factory constructor, and have it
simply construct the appropriate subtype. This is simpler, and should
be much lower risk, since we no longer have a code path that has to
throw an exception.

Change-Id: Ie3ed216b285e371866a6a35ccccdf997b8a1f95e
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/396620
Reviewed-by: Brian Wilkerson <[email protected]>
Reviewed-by: Konstantin Shcheglov <[email protected]>
Commit-Queue: Paul Berry <[email protected]>

Author: stereotype441

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

@@ -5913,16 +5913,11 @@ abstract class InterfaceElementImpl extends InstanceElementImpl
       }
     }
 
-    InterfaceTypeImpl result;
-    if (name == 'Null' && library.isDartCore) {
-      result = NullTypeImpl(element: this);
-    } else {
-      result = InterfaceTypeImpl(
-        element: this,
-        typeArguments: typeArguments,
-        nullabilitySuffix: nullabilitySuffix,
-      );
-    }
+    var result = InterfaceTypeImpl(
+      element: this,
+      typeArguments: typeArguments,
+      nullabilitySuffix: nullabilitySuffix,
+    );
 
     if (typeArguments.isEmpty) {
       switch (nullabilitySuffix) {
@@ -10283,11 +10278,6 @@ class TypeAliasElementImpl extends _ExistingElementImpl
           typeArguments: typeArguments,
         ),
       );
-    } else if (type is NullTypeImpl) {
-      return NullTypeImpl(
-          element: type.element,
-          alias: InstantiatedTypeAliasElementImpl(
-              element: this, typeArguments: typeArguments));
     } else if (type is InterfaceType) {
       return InterfaceTypeImpl(
         element: type.element,

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

@@ -534,6 +534,8 @@ class InterfaceTypeImpl extends TypeImpl implements InterfaceType {
               : InvalidTypeImpl.instance,
           nullabilitySuffix: nullabilitySuffix,
           alias: alias);
+    } else if (element.name == 'Null' && element.library.isDartCore) {
+      return NullTypeImpl(element: element, alias: alias);
     } else {
       return InterfaceTypeImpl._(
           element: element,
@@ -549,9 +551,6 @@ class InterfaceTypeImpl extends TypeImpl implements InterfaceType {
     required this.nullabilitySuffix,
     required super.alias,
   }) {
-    if (element.name == 'Null' && element.library.isDartCore) {
-      throw ArgumentError('NullTypeImpl should be used for `Null`');
-    }
     if (element.augmentationTarget != null) {
       throw ArgumentError(
         'InterfaceType(s) can only be created for declarations',

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

@@ -608,14 +608,10 @@ class TypeProviderImpl extends TypeProviderBase {
       }).toList(growable: false);
     }
 
-    if (element.name == 'Null' && element.library.isDartCore) {
-      return NullTypeImpl(element: element);
-    } else {
-      return InterfaceTypeImpl(
-        element: element,
-        typeArguments: typeArguments,
-        nullabilitySuffix: NullabilitySuffix.none,
-      );
-    }
+    return InterfaceTypeImpl(
+      element: element,
+      typeArguments: typeArguments,
+      nullabilitySuffix: NullabilitySuffix.none,
+    );
   }
 }