diff --git a/pkg/analysis_server/lib/src/services/completion/dart/not_imported_contributor.dart b/pkg/analysis_server/lib/src/services/completion/dart/not_imported_contributor.dart
index d814bb5..8bbff4f 100644
--- a/pkg/analysis_server/lib/src/services/completion/dart/not_imported_contributor.dart
+++ b/pkg/analysis_server/lib/src/services/completion/dart/not_imported_contributor.dart
@@ -92,7 +92,7 @@
       }
 
       extensionContributor.addExtensions(
-        exportElements.whereType<ExtensionElement>().toList(),
+        _extensions(exportElements),
       );
 
       builder.isNotImportedLibrary = false;
@@ -109,4 +109,17 @@
       }
     }
   }
+
+  /// This function intentionally does not use `whereType` for performance.
+  ///
+  /// https://github.com/dart-lang/sdk/issues/47680
+  static List<ExtensionElement> _extensions(List<Element> elements) {
+    var extensions = <ExtensionElement>[];
+    for (var element in elements) {
+      if (element is ExtensionElement) {
+        extensions.add(element);
+      }
+    }
+    return extensions;
+  }
 }
diff --git a/pkg/analyzer/lib/src/dart/ast/extensions.dart b/pkg/analyzer/lib/src/dart/ast/extensions.dart
index 7788785..3df6b5d 100644
--- a/pkg/analyzer/lib/src/dart/ast/extensions.dart
+++ b/pkg/analyzer/lib/src/dart/ast/extensions.dart
@@ -76,6 +76,21 @@
   return null;
 }
 
+extension AstNodeNullableExtension on AstNode? {
+  List<ClassMember> get classMembers {
+    final self = this;
+    if (self is ClassOrMixinDeclaration) {
+      return self.members;
+    } else if (self is EnumDeclaration) {
+      return self.members;
+    } else if (self is ExtensionDeclaration) {
+      return self.members;
+    } else {
+      throw UnimplementedError('(${self.runtimeType}) $self');
+    }
+  }
+}
+
 extension ConstructorDeclarationExtension on ConstructorDeclaration {
   bool get isNonRedirectingGenerative {
     // Must be generative.
diff --git a/pkg/analyzer/lib/src/dart/constant/constant_verifier.dart b/pkg/analyzer/lib/src/dart/constant/constant_verifier.dart
index bcbe8fa..9af29c4 100644
--- a/pkg/analyzer/lib/src/dart/constant/constant_verifier.dart
+++ b/pkg/analyzer/lib/src/dart/constant/constant_verifier.dart
@@ -103,8 +103,7 @@
     if (constKeyword != null) {
       _validateConstructorInitializers(node);
       if (node.factoryKeyword == null) {
-        _validateFieldInitializers(
-            node.parent as ClassOrMixinDeclaration, constKeyword);
+        _validateFieldInitializers(node.parent.classMembers, constKeyword);
       }
     }
     _validateDefaultValues(node.parameters);
@@ -545,12 +544,11 @@
   }
 
   /// Validates that the expressions of any field initializers in
-  /// [classDeclaration] are all compile-time constants. Since this is only
+  /// [members] are all compile-time constants. Since this is only
   /// required if the class has a constant constructor, the error is reported at
   /// [constKeyword], the const keyword on such a constant constructor.
   void _validateFieldInitializers(
-      ClassOrMixinDeclaration classDeclaration, Token constKeyword) {
-    NodeList<ClassMember> members = classDeclaration.members;
+      List<ClassMember> members, Token constKeyword) {
     for (ClassMember member in members) {
       if (member is FieldDeclaration && !member.isStatic) {
         for (VariableDeclaration variableDeclaration
diff --git a/pkg/analyzer/lib/src/generated/error_verifier.dart b/pkg/analyzer/lib/src/generated/error_verifier.dart
index 56a90a4..9176c6e 100644
--- a/pkg/analyzer/lib/src/generated/error_verifier.dart
+++ b/pkg/analyzer/lib/src/generated/error_verifier.dart
@@ -202,11 +202,7 @@
 
   /// The class containing the AST nodes being visited, or `null` if we are not
   /// in the scope of a class.
-  ClassElementImpl? _enclosingClass;
-
-  /// The enum containing the AST nodes being visited, or `null` if we are not
-  /// in the scope of an enum.
-  ClassElement? _enclosingEnum;
+  ClassElement? _enclosingClass;
 
   /// The element of the extension being visited, or `null` if we are not
   /// in the scope of an extension.
@@ -281,11 +277,8 @@
   /// should not be modified in the middle of visiting a tree and requires an
   /// analyzer-provided Impl instance to work.
   set enclosingClass(ClassElement? classElement) {
-    assert(classElement is ClassElementImpl);
     assert(_enclosingClass == null);
-    assert(_enclosingEnum == null);
     assert(_enclosingExecutable.element == null);
-    _enclosingClass = classElement as ClassElementImpl;
   }
 
   /// The language team is thinking about adding abstract fields, or external
@@ -563,13 +556,13 @@
 
   @override
   void visitEnumDeclaration(EnumDeclaration node) {
-    var outerEnum = _enclosingEnum;
+    var outerClass = _enclosingClass;
     try {
-      _enclosingEnum = node.declaredElement;
+      _enclosingClass = node.declaredElement;
       _duplicateDefinitionVerifier.checkEnum(node);
       super.visitEnumDeclaration(node);
     } finally {
-      _enclosingEnum = outerEnum;
+      _enclosingClass = outerClass;
     }
   }
 
@@ -4408,9 +4401,6 @@
     if (identical(enclosingElement, _enclosingClass)) {
       return;
     }
-    if (identical(enclosingElement, _enclosingEnum)) {
-      return;
-    }
     if (enclosingElement is! ClassElement) {
       return;
     }
diff --git a/tools/VERSION b/tools/VERSION
index 5d43e34..9c19b90 100644
--- a/tools/VERSION
+++ b/tools/VERSION
@@ -27,5 +27,5 @@
 MAJOR 2
 MINOR 17
 PATCH 0
-PRERELEASE 56
+PRERELEASE 57
 PRERELEASE_PATCH 0
\ No newline at end of file