Elements. Migrate BaseOrFinalTypeVerifier.

Change-Id: I0fd47be98550d288d3ff1e5fb4c148150ab727b9
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/395162
Reviewed-by: Brian Wilkerson <[email protected]>
Commit-Queue: Konstantin Shcheglov <[email protected]>

Author: scheglov

pkg/analyzer/analyzer_use_new_elements.txt

@@ -99,7 +99,6 @@ lib/src/dart/resolver/variable_declaration_resolver.dart
 lib/src/dart/resolver/yield_statement_resolver.dart
 lib/src/diagnostic/diagnostic_factory.dart
 lib/src/error/assignment_verifier.dart
-lib/src/error/base_or_final_type_verifier.dart
 lib/src/error/best_practices_verifier.dart
 lib/src/error/correct_override.dart
 lib/src/error/dead_code_verifier.dart

pkg/analyzer/lib/src/dart/ast/ast.dart

@@ -2845,7 +2845,7 @@ final class ClassDeclarationImpl extends NamedCompilationUnitMemberImpl
 
   @experimental
   @override
-  ClassFragment? get declaredFragment => declaredElement as ClassFragment;
+  ClassElementImpl? get declaredFragment => declaredElement;
 
   @override
   Token get endToken => rightBracket;
@@ -3052,7 +3052,7 @@ final class ClassTypeAliasImpl extends TypeAliasImpl implements ClassTypeAlias {
 
   @experimental
   @override
-  ClassFragment? get declaredFragment => declaredElement as ClassFragment?;
+  ClassElementImpl? get declaredFragment => declaredElement;
 
   @override
   Token get firstTokenAfterCommentAndMetadata {
@@ -12267,7 +12267,7 @@ final class MixinDeclarationImpl extends NamedCompilationUnitMemberImpl
 
   @experimental
   @override
-  MixinFragment? get declaredFragment => declaredElement as MixinFragment?;
+  MixinElementImpl? get declaredFragment => declaredElement;
 
   @override
   Token get endToken => rightBracket;