diff --git a/pkg/front_end/lib/src/fragment/constructor/declaration.dart b/pkg/front_end/lib/src/fragment/constructor/declaration.dart
index 8c37caca0db..36c0ce90b3f 100644
--- a/pkg/front_end/lib/src/fragment/constructor/declaration.dart
+++ b/pkg/front_end/lib/src/fragment/constructor/declaration.dart
@@ -31,7 +31,6 @@ import '../../source/check_helper.dart';
 import '../../source/name_scheme.dart';
 import '../../source/source_class_builder.dart';
 import '../../source/source_constructor_builder.dart';
-import '../../source/source_extension_type_declaration_builder.dart';
 import '../../source/source_function_builder.dart';
 import '../../source/source_library_builder.dart';
 import '../../source/source_loader.dart';
@@ -1337,9 +1336,7 @@ class PrimaryConstructorDeclaration
       f,
       constructorBuilder: constructorBuilder,
       libraryBuilder: libraryBuilder,
-      declarationBuilder:
-          constructorBuilder.declarationBuilder
-              as SourceExtensionTypeDeclarationBuilder,
+      declarationBuilder: constructorBuilder.declarationBuilder,
       name: _fragment.name,
       nameScheme: nameScheme,
       constructorReferences: constructorReferences,
diff --git a/pkg/front_end/lib/src/source/outline_builder.dart b/pkg/front_end/lib/src/source/outline_builder.dart
index 236266067f4..87705593869 100644
--- a/pkg/front_end/lib/src/source/outline_builder.dart
+++ b/pkg/front_end/lib/src/source/outline_builder.dart
@@ -1825,123 +1825,126 @@ class OutlineBuilder extends StackListenerImpl {
       name = identifier.name;
     }
 
-    if (!forExtensionType) {
+    int? startOffset = constKeyword?.charOffset ?? nameOffset ?? formalsOffset;
+
+    // TODO(johnniwinther): Handle declaring parameters.
+    if (forExtensionType) {
+      bool inExtensionType =
+          declarationContext == DeclarationContext.ExtensionType;
+      if (formals != null) {
+        int requiredPositionalCount = 0;
+        int? firstNamedParameterOffset;
+        int? firstOptionalPositionalParameterOffset;
+        for (int i = 0; i < formals.length; i++) {
+          FormalParameterBuilder formal = formals[i];
+          if (inExtensionType) {
+            TypeBuilder type = formal.type;
+            if (type is FunctionTypeBuilder &&
+                type.hasFunctionFormalParameterSyntax) {
+              _compilationUnit.addProblem(
+                // ignore: lines_longer_than_80_chars
+                codeExtensionTypePrimaryConstructorFunctionFormalParameterSyntax,
+                formal.fileOffset,
+                formal.name.length,
+                formal.fileUri,
+              );
+            }
+            if (type is ImplicitTypeBuilder) {
+              _compilationUnit.addProblem(
+                codeExpectedRepresentationType,
+                formal.fileOffset,
+                formal.name.length,
+                formal.fileUri,
+              );
+              formal.type = new InvalidTypeBuilderImpl(
+                formal.fileUri,
+                formal.fileOffset,
+              );
+            }
+            if (formal.modifiers.containsSyntacticModifiers(
+              ignoreCovariant: true,
+              ignoreRequired: true,
+            )) {
+              _compilationUnit.addProblem(
+                codeRepresentationFieldModifier,
+                formal.fileOffset,
+                formal.name.length,
+                formal.fileUri,
+              );
+            }
+            if (formal.isInitializingFormal) {
+              _compilationUnit.addProblem(
+                codeExtensionTypePrimaryConstructorWithInitializingFormal,
+                formal.fileOffset,
+                formal.name.length,
+                formal.fileUri,
+              );
+            }
+          }
+
+          if (formal.isPositional) {
+            if (formal.isOptionalPositional) {
+              firstOptionalPositionalParameterOffset = formal.fileOffset;
+            } else {
+              requiredPositionalCount++;
+            }
+          }
+          if (formal.isNamed) {
+            firstNamedParameterOffset = formal.fileOffset;
+          }
+          _builderFactory.addPrimaryConstructorField(
+            // TODO(johnniwinther): Support annotations on annotations on fields
+            // defined through a primary constructor. This is not needed for
+            // extension types where the field is not part of the AST but will
+            // be needed when primary constructors are generally supported.
+            metadata: null,
+            type: formal.type,
+            name: formal.name,
+            nameOffset: formal.fileOffset,
+          );
+          formals[i] = formal.forPrimaryConstructor(_builderFactory);
+        }
+        if (inExtensionType) {
+          if (firstOptionalPositionalParameterOffset != null) {
+            _compilationUnit.addProblem(
+              codeOptionalParametersInExtensionTypeDeclaration,
+              firstOptionalPositionalParameterOffset,
+              1,
+              uri,
+            );
+          } else if (firstNamedParameterOffset != null) {
+            _compilationUnit.addProblem(
+              codeNamedParametersInExtensionTypeDeclaration,
+              firstNamedParameterOffset,
+              1,
+              uri,
+            );
+          } else if (requiredPositionalCount == 0) {
+            _compilationUnit.addProblem(
+              codeExpectedRepresentationField,
+              charOffset,
+              1,
+              uri,
+            );
+          } else if (formals.length > 1) {
+            _compilationUnit.addProblem(
+              codeMultipleRepresentationFields,
+              charOffset,
+              1,
+              uri,
+            );
+          }
+        }
+      }
+    } else {
       reportIfNotEnabled(
         libraryFeatures.declaringConstructors,
         beginToken.charOffset,
         noLength,
       );
-      // TODO(johnniwinther): Support primary constructors in general.
-      return;
-    }
-
-    int? startOffset = constKeyword?.charOffset ?? nameOffset ?? formalsOffset;
-
-    bool inExtensionType =
-        declarationContext == DeclarationContext.ExtensionType;
-    if (formals != null) {
-      int requiredPositionalCount = 0;
-      int? firstNamedParameterOffset;
-      int? firstOptionalPositionalParameterOffset;
-      for (int i = 0; i < formals.length; i++) {
-        FormalParameterBuilder formal = formals[i];
-        if (inExtensionType) {
-          TypeBuilder type = formal.type;
-          if (type is FunctionTypeBuilder &&
-              type.hasFunctionFormalParameterSyntax) {
-            _compilationUnit.addProblem(
-              // ignore: lines_longer_than_80_chars
-              codeExtensionTypePrimaryConstructorFunctionFormalParameterSyntax,
-              formal.fileOffset,
-              formal.name.length,
-              formal.fileUri,
-            );
-          }
-          if (type is ImplicitTypeBuilder) {
-            _compilationUnit.addProblem(
-              codeExpectedRepresentationType,
-              formal.fileOffset,
-              formal.name.length,
-              formal.fileUri,
-            );
-            formal.type = new InvalidTypeBuilderImpl(
-              formal.fileUri,
-              formal.fileOffset,
-            );
-          }
-          if (formal.modifiers.containsSyntacticModifiers(
-            ignoreCovariant: true,
-            ignoreRequired: true,
-          )) {
-            _compilationUnit.addProblem(
-              codeRepresentationFieldModifier,
-              formal.fileOffset,
-              formal.name.length,
-              formal.fileUri,
-            );
-          }
-          if (formal.isInitializingFormal) {
-            _compilationUnit.addProblem(
-              codeExtensionTypePrimaryConstructorWithInitializingFormal,
-              formal.fileOffset,
-              formal.name.length,
-              formal.fileUri,
-            );
-          }
-        }
-
-        if (formal.isPositional) {
-          if (formal.isOptionalPositional) {
-            firstOptionalPositionalParameterOffset = formal.fileOffset;
-          } else {
-            requiredPositionalCount++;
-          }
-        }
-        if (formal.isNamed) {
-          firstNamedParameterOffset = formal.fileOffset;
-        }
-        _builderFactory.addPrimaryConstructorField(
-          // TODO(johnniwinther): Support annotations on annotations on fields
-          // defined through a primary constructor. This is not needed for
-          // extension types where the field is not part of the AST but will
-          // be needed when primary constructors are generally supported.
-          metadata: null,
-          type: formal.type,
-          name: formal.name,
-          nameOffset: formal.fileOffset,
-        );
-        formals[i] = formal.forPrimaryConstructor(_builderFactory);
-      }
-      if (inExtensionType) {
-        if (firstOptionalPositionalParameterOffset != null) {
-          _compilationUnit.addProblem(
-            codeOptionalParametersInExtensionTypeDeclaration,
-            firstOptionalPositionalParameterOffset,
-            1,
-            uri,
-          );
-        } else if (firstNamedParameterOffset != null) {
-          _compilationUnit.addProblem(
-            codeNamedParametersInExtensionTypeDeclaration,
-            firstNamedParameterOffset,
-            1,
-            uri,
-          );
-        } else if (requiredPositionalCount == 0) {
-          _compilationUnit.addProblem(
-            codeExpectedRepresentationField,
-            charOffset,
-            1,
-            uri,
-          );
-        } else if (formals.length > 1) {
-          _compilationUnit.addProblem(
-            codeMultipleRepresentationFields,
-            charOffset,
-            1,
-            uri,
-          );
-        }
+      if (declarationContext == DeclarationContext.Enum) {
+        // TODO(johnniwinther): Support primary constructors in enums.
+        return;
       }
     }
 
diff --git a/pkg/front_end/testcases/declaring_constructors/class_primary_constructor.dart b/pkg/front_end/testcases/declaring_constructors/class_primary_constructor.dart
new file mode 100644
index 00000000000..f5c01432ff6
--- /dev/null
+++ b/pkg/front_end/testcases/declaring_constructors/class_primary_constructor.dart
@@ -0,0 +1,15 @@
+// Copyright (c) 2025, the Dart project authors. Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+class C1() {}
+
+class const C2() {}
+
+class C3() {
+  final int? i; // Error
+}
+
+class const C4() { // Error
+  int? i;
+}
diff --git a/pkg/front_end/testcases/declaring_constructors/class_primary_constructor.dart.strong.expect b/pkg/front_end/testcases/declaring_constructors/class_primary_constructor.dart.strong.expect
new file mode 100644
index 00000000000..0fb3b502359
--- /dev/null
+++ b/pkg/front_end/testcases/declaring_constructors/class_primary_constructor.dart.strong.expect
@@ -0,0 +1,41 @@
+library;
+//
+// Problems in library:
+//
+// pkg/front_end/testcases/declaring_constructors/class_primary_constructor.dart:10:14: Error: Final field 'i' is not initialized.
+// Try to initialize the field in the declaration or in every constructor.
+//   final int? i; // Error
+//              ^
+//
+// pkg/front_end/testcases/declaring_constructors/class_primary_constructor.dart:13:15: Error: Constructor is marked 'const' so all fields must be final.
+// class const C4() { // Error
+//               ^
+// pkg/front_end/testcases/declaring_constructors/class_primary_constructor.dart:14:8: Context: Field isn't final, but constructor is 'const'.
+//   int? i;
+//        ^
+//
+import self as self;
+import "dart:core" as core;
+
+class C1 extends core::Object {
+  constructor •() → self::C1
+    : super core::Object::•()
+    ;
+}
+class C2 extends core::Object /*hasConstConstructor*/  {
+  const constructor •() → self::C2
+    : super core::Object::•()
+    ;
+}
+class C3 extends core::Object {
+  final field core::int? i = null;
+  constructor •() → self::C3
+    : super core::Object::•()
+    ;
+}
+class C4 extends core::Object /*hasConstConstructor*/  {
+  field core::int? i = null;
+  const constructor •() → self::C4
+    : super core::Object::•()
+    ;
+}
diff --git a/pkg/front_end/testcases/declaring_constructors/class_primary_constructor.dart.strong.modular.expect b/pkg/front_end/testcases/declaring_constructors/class_primary_constructor.dart.strong.modular.expect
new file mode 100644
index 00000000000..0fb3b502359
--- /dev/null
+++ b/pkg/front_end/testcases/declaring_constructors/class_primary_constructor.dart.strong.modular.expect
@@ -0,0 +1,41 @@
+library;
+//
+// Problems in library:
+//
+// pkg/front_end/testcases/declaring_constructors/class_primary_constructor.dart:10:14: Error: Final field 'i' is not initialized.
+// Try to initialize the field in the declaration or in every constructor.
+//   final int? i; // Error
+//              ^
+//
+// pkg/front_end/testcases/declaring_constructors/class_primary_constructor.dart:13:15: Error: Constructor is marked 'const' so all fields must be final.
+// class const C4() { // Error
+//               ^
+// pkg/front_end/testcases/declaring_constructors/class_primary_constructor.dart:14:8: Context: Field isn't final, but constructor is 'const'.
+//   int? i;
+//        ^
+//
+import self as self;
+import "dart:core" as core;
+
+class C1 extends core::Object {
+  constructor •() → self::C1
+    : super core::Object::•()
+    ;
+}
+class C2 extends core::Object /*hasConstConstructor*/  {
+  const constructor •() → self::C2
+    : super core::Object::•()
+    ;
+}
+class C3 extends core::Object {
+  final field core::int? i = null;
+  constructor •() → self::C3
+    : super core::Object::•()
+    ;
+}
+class C4 extends core::Object /*hasConstConstructor*/  {
+  field core::int? i = null;
+  const constructor •() → self::C4
+    : super core::Object::•()
+    ;
+}
diff --git a/pkg/front_end/testcases/declaring_constructors/class_primary_constructor.dart.strong.outline.expect b/pkg/front_end/testcases/declaring_constructors/class_primary_constructor.dart.strong.outline.expect
new file mode 100644
index 00000000000..ba997af5616
--- /dev/null
+++ b/pkg/front_end/testcases/declaring_constructors/class_primary_constructor.dart.strong.outline.expect
@@ -0,0 +1,24 @@
+library;
+import self as self;
+import "dart:core" as core;
+
+class C1 extends core::Object {
+  constructor •() → self::C1
+    ;
+}
+class C2 extends core::Object /*hasConstConstructor*/  {
+  const constructor •() → self::C2
+    : super core::Object::•()
+    ;
+}
+class C3 extends core::Object {
+  final field core::int? i;
+  constructor •() → self::C3
+    ;
+}
+class C4 extends core::Object /*hasConstConstructor*/  {
+  field core::int? i;
+  const constructor •() → self::C4
+    : super core::Object::•()
+    ;
+}
diff --git a/pkg/front_end/testcases/declaring_constructors/class_primary_constructor.dart.strong.transformed.expect b/pkg/front_end/testcases/declaring_constructors/class_primary_constructor.dart.strong.transformed.expect
new file mode 100644
index 00000000000..0fb3b502359
--- /dev/null
+++ b/pkg/front_end/testcases/declaring_constructors/class_primary_constructor.dart.strong.transformed.expect
@@ -0,0 +1,41 @@
+library;
+//
+// Problems in library:
+//
+// pkg/front_end/testcases/declaring_constructors/class_primary_constructor.dart:10:14: Error: Final field 'i' is not initialized.
+// Try to initialize the field in the declaration or in every constructor.
+//   final int? i; // Error
+//              ^
+//
+// pkg/front_end/testcases/declaring_constructors/class_primary_constructor.dart:13:15: Error: Constructor is marked 'const' so all fields must be final.
+// class const C4() { // Error
+//               ^
+// pkg/front_end/testcases/declaring_constructors/class_primary_constructor.dart:14:8: Context: Field isn't final, but constructor is 'const'.
+//   int? i;
+//        ^
+//
+import self as self;
+import "dart:core" as core;
+
+class C1 extends core::Object {
+  constructor •() → self::C1
+    : super core::Object::•()
+    ;
+}
+class C2 extends core::Object /*hasConstConstructor*/  {
+  const constructor •() → self::C2
+    : super core::Object::•()
+    ;
+}
+class C3 extends core::Object {
+  final field core::int? i = null;
+  constructor •() → self::C3
+    : super core::Object::•()
+    ;
+}
+class C4 extends core::Object /*hasConstConstructor*/  {
+  field core::int? i = null;
+  const constructor •() → self::C4
+    : super core::Object::•()
+    ;
+}
diff --git a/pkg/front_end/testcases/declaring_constructors/class_primary_constructor.dart.textual_outline.expect b/pkg/front_end/testcases/declaring_constructors/class_primary_constructor.dart.textual_outline.expect
new file mode 100644
index 00000000000..bed589dc0e1
--- /dev/null
+++ b/pkg/front_end/testcases/declaring_constructors/class_primary_constructor.dart.textual_outline.expect
@@ -0,0 +1,11 @@
+class C1() {}
+
+class const C2() {}
+
+class C3() {
+  final int? i;
+}
+
+class const C4() {
+  int? i;
+}
diff --git a/pkg/front_end/testcases/declaring_constructors/class_primary_constructor.dart.textual_outline_modelled.expect b/pkg/front_end/testcases/declaring_constructors/class_primary_constructor.dart.textual_outline_modelled.expect
new file mode 100644
index 00000000000..1eaf42f3f78
--- /dev/null
+++ b/pkg/front_end/testcases/declaring_constructors/class_primary_constructor.dart.textual_outline_modelled.expect
@@ -0,0 +1,11 @@
+class C1() {}
+
+class C3() {
+  final int? i;
+}
+
+class const C2() {}
+
+class const C4() {
+  int? i;
+}
diff --git a/pkg/front_end/testcases/extension_types/with_dependencies/issue53209_2/main.dart.strong.expect b/pkg/front_end/testcases/extension_types/with_dependencies/issue53209_2/main.dart.strong.expect
index 1d19136490d..6197de3c01d 100644
--- a/pkg/front_end/testcases/extension_types/with_dependencies/issue53209_2/main.dart.strong.expect
+++ b/pkg/front_end/testcases/extension_types/with_dependencies/issue53209_2/main.dart.strong.expect
@@ -18,6 +18,9 @@ import "dart:core" as core;
 import "org-dartlang-testcase:///main_lib.dart";
 
 class A extends core::Object {
+  constructor •(core::String foo) → self::A
+    : super core::Object::•()
+    ;
   static factory redirectingFactoryToSubtype() → self::A
     return invalid-expression "pkg/front_end/testcases/extension_types/with_dependencies/issue53209_2/main.dart:8:45: Error: The constructor function type 'E Function()' isn't a subtype of 'A Function()'.
  - 'A' is from 'pkg/front_end/testcases/extension_types/with_dependencies/issue53209_2/main.dart'.
diff --git a/pkg/front_end/testcases/extension_types/with_dependencies/issue53209_2/main.dart.strong.modular.expect b/pkg/front_end/testcases/extension_types/with_dependencies/issue53209_2/main.dart.strong.modular.expect
index 0a58f090b50..d713dae1642 100644
--- a/pkg/front_end/testcases/extension_types/with_dependencies/issue53209_2/main.dart.strong.modular.expect
+++ b/pkg/front_end/testcases/extension_types/with_dependencies/issue53209_2/main.dart.strong.modular.expect
@@ -18,6 +18,9 @@ import "dart:core" as core;
 import "org-dartlang-testcase:///main_lib.dart";
 
 class A extends core::Object {
+  constructor •(core::String foo) → self::A
+    : super core::Object::•()
+    ;
   static factory redirectingFactoryToSubtype() → self::A
     return invalid-expression "pkg/front_end/testcases/extension_types/with_dependencies/issue53209_2/main.dart:8:45: Error: The constructor function type 'E Function()' isn't a subtype of 'A Function()'.
  - 'A' is from 'pkg/front_end/testcases/extension_types/with_dependencies/issue53209_2/main.dart'.
diff --git a/pkg/front_end/testcases/extension_types/with_dependencies/issue53209_2/main.dart.strong.outline.expect b/pkg/front_end/testcases/extension_types/with_dependencies/issue53209_2/main.dart.strong.outline.expect
index 701c51f3b83..6c745939d25 100644
--- a/pkg/front_end/testcases/extension_types/with_dependencies/issue53209_2/main.dart.strong.outline.expect
+++ b/pkg/front_end/testcases/extension_types/with_dependencies/issue53209_2/main.dart.strong.outline.expect
@@ -18,6 +18,8 @@ import "dart:core" as core;
 import "org-dartlang-testcase:///main_lib.dart";
 
 class A extends core::Object {
+  constructor •(core::String foo) → self::A
+    ;
   static factory redirectingFactoryToSubtype() → self::A
     return invalid-expression "pkg/front_end/testcases/extension_types/with_dependencies/issue53209_2/main.dart:8:45: Error: The constructor function type 'E Function()' isn't a subtype of 'A Function()'.
  - 'A' is from 'pkg/front_end/testcases/extension_types/with_dependencies/issue53209_2/main.dart'.
diff --git a/pkg/front_end/testcases/extension_types/with_dependencies/issue53209_2/main.dart.strong.transformed.expect b/pkg/front_end/testcases/extension_types/with_dependencies/issue53209_2/main.dart.strong.transformed.expect
index 1d19136490d..6197de3c01d 100644
--- a/pkg/front_end/testcases/extension_types/with_dependencies/issue53209_2/main.dart.strong.transformed.expect
+++ b/pkg/front_end/testcases/extension_types/with_dependencies/issue53209_2/main.dart.strong.transformed.expect
@@ -18,6 +18,9 @@ import "dart:core" as core;
 import "org-dartlang-testcase:///main_lib.dart";
 
 class A extends core::Object {
+  constructor •(core::String foo) → self::A
+    : super core::Object::•()
+    ;
   static factory redirectingFactoryToSubtype() → self::A
     return invalid-expression "pkg/front_end/testcases/extension_types/with_dependencies/issue53209_2/main.dart:8:45: Error: The constructor function type 'E Function()' isn't a subtype of 'A Function()'.
  - 'A' is from 'pkg/front_end/testcases/extension_types/with_dependencies/issue53209_2/main.dart'.