diff --git a/README.md b/README.md
index bdd79718..4f0a6057 100644
--- a/README.md
+++ b/README.md
@@ -1,15 +1,19 @@
+This repository provides the Dart packages `reflectable` and
+`reflectable_builder`, along with a set of test cases and
+examples in the repository `test_reflectable`.
 
-This repository provides the Dart package `reflectable` along with a set of
-test cases, `test_reflectable`.
+## [reflectable](https://github.com/google/reflectable.dart/blob/master/packages/reflectable/README.md)
 
-## [reflectable](https://github.com/google/reflectable.dart/blob/master/reflectable/README.md)
+Support for implementing a large subset of the features offered by
+'dart:mirrors' without relying on 'dart:mirrors' itself.  Provides a system
+based on capabilities to control the amount of reflection support.
 
-Support for generating code that implements a large subset of the features
-offered by 'dart:mirrors' without relying on 'dart:mirrors' itself.
-Provides a system based on capabilities to control the amount of reflection
-support.
+## [reflectable_builder](https://github.com/google/reflectable.dart/blob/master/packages/reflectable_builder/README.md)
 
-## [test_reflectable](https://github.com/google/reflectable.dart/blob/master/test_reflectable/README.md)
+Support for generating the code that the package `reflectable` uses to provide
+the reflection support described above.
+
+## [test_reflectable](https://github.com/google/reflectable.dart/blob/master/packages/test_reflectable/README.md)
 
 Used to test package `reflectable`. Also serves as a set of examples of how
 `reflectable` can be used.
diff --git a/packages/reflectable/CHANGELOG.md b/packages/reflectable/CHANGELOG.md
index 7b83f0b7..858b2253 100644
--- a/packages/reflectable/CHANGELOG.md
+++ b/packages/reflectable/CHANGELOG.md
@@ -1,3 +1,7 @@
+## 5.2.1
+
+- Fix bugs #354 and #325.
+
 ## 5.2.0
 
 - Upgrade reflectable to use analyzer ^10.0.0 and lints ^6.0.0.
diff --git a/packages/reflectable/example/bin/example.dart b/packages/reflectable/example/bin/example.dart
index 06dd352d..4d5998ea 100644
--- a/packages/reflectable/example/bin/example.dart
+++ b/packages/reflectable/example/bin/example.dart
@@ -22,7 +22,7 @@ class A {
   int argNamed(int x, int y, {int z = 42}) => x + y - z;
   int operator +(int x) => 42 + x;
   int operator [](int x) => 42 + x;
-  void operator []=(x, v) {
+  void operator []=(int x, int v) {
     f = x + v;
   }
 
@@ -32,8 +32,9 @@ class A {
   int f = 0;
 
   static int noArguments() => 42;
-  static int oneArgument(x) => x - 42;
-  static int optionalArguments(x, y, [z = 0, w]) => x + y + z * 42;
+  static int oneArgument(int x) => x - 42;
+  static int optionalArguments(int x, int y, [int z = 0, int? w]) =>
+      x + y + z * 42;
   static int namedArguments(int x, int y, {int z = 42}) => x + y - z;
 }
 
diff --git a/packages/reflectable/pubspec.yaml b/packages/reflectable/pubspec.yaml
index 321c7360..a80aacdf 100644
--- a/packages/reflectable/pubspec.yaml
+++ b/packages/reflectable/pubspec.yaml
@@ -1,5 +1,5 @@
 name: reflectable
-version: 5.2.0
+version: 5.2.1
 description: >
   Reflection support based on code generation, using 'capabilities' to
   specify which operations to support, on which objects. This is the
diff --git a/packages/reflectable_builder/CHANGELOG.md b/packages/reflectable_builder/CHANGELOG.md
index d96c7b7c..42378eb9 100644
--- a/packages/reflectable_builder/CHANGELOG.md
+++ b/packages/reflectable_builder/CHANGELOG.md
@@ -1,3 +1,7 @@
+## 1.2.1
+
+- Fix bugs #354 and #325.
+
 ## 1.2.0
 
 - Upgrade the code generator to use analyzer ^10.0.0 and lints 6.0.0.
diff --git a/packages/reflectable_builder/example/example.dart b/packages/reflectable_builder/example/example.dart
index 06dd352d..4d5998ea 100644
--- a/packages/reflectable_builder/example/example.dart
+++ b/packages/reflectable_builder/example/example.dart
@@ -22,7 +22,7 @@ class A {
   int argNamed(int x, int y, {int z = 42}) => x + y - z;
   int operator +(int x) => 42 + x;
   int operator [](int x) => 42 + x;
-  void operator []=(x, v) {
+  void operator []=(int x, int v) {
     f = x + v;
   }
 
@@ -32,8 +32,9 @@ class A {
   int f = 0;
 
   static int noArguments() => 42;
-  static int oneArgument(x) => x - 42;
-  static int optionalArguments(x, y, [z = 0, w]) => x + y + z * 42;
+  static int oneArgument(int x) => x - 42;
+  static int optionalArguments(int x, int y, [int z = 0, int? w]) =>
+      x + y + z * 42;
   static int namedArguments(int x, int y, {int z = 42}) => x + y - z;
 }
 
diff --git a/packages/reflectable_builder/lib/src/builder_implementation.dart b/packages/reflectable_builder/lib/src/builder_implementation.dart
index cf20dfca..9753acc1 100644
--- a/packages/reflectable_builder/lib/src/builder_implementation.dart
+++ b/packages/reflectable_builder/lib/src/builder_implementation.dart
@@ -4897,6 +4897,30 @@ class BuilderImplementation {
     }
     imports.sort();
 
+    // Some "private" platform libraries are exported by public ones. To avoid
+    // the error caused by importing 'dart:_http', we transform it to 'dart:io'.
+    // Other cases are ambiguous and not handled.
+    final length = imports.length;
+    for (int index = 0; index < length; ++index) {
+      final import = imports[index];
+      if (import.contains("'dart:_")) {
+        if (import.contains("'dart:_http'")) {
+          final rest = import.substring(
+            "import 'dart:_http' ".length,
+            import.length,
+          );
+          imports[index] = "import 'dart:io' $rest";
+        } else {
+          // TODO(eernst): Perhaps we can handle some of the remaining cases.
+          final match = RegExp(r"'dart:_.*'").firstMatch(import);
+          final String name = match != null
+              ? import.substring(match.start, match.end)
+              : import;
+          await _severe('Reference to an internal platform library: $name');
+        }
+      }
+    }
+
     var result =
         '''
 // This file has been generated by the reflectable package.
@@ -5575,6 +5599,7 @@ const Set<String> sdkLibraryNames = <String>{
   'web_audio',
   'web_gl',
   'web_sql',
+  '_http', // Special case, redirected to 'dart:io'.
 };
 
 // Helper for _extractMetadataCode.
diff --git a/packages/reflectable_builder/pubspec.yaml b/packages/reflectable_builder/pubspec.yaml
index 4134ed1a..379cac6b 100644
--- a/packages/reflectable_builder/pubspec.yaml
+++ b/packages/reflectable_builder/pubspec.yaml
@@ -1,5 +1,5 @@
 name: reflectable_builder
-version: 1.2.0
+version: 1.2.1
 description: >
   Reflection support based on code generation, using 'capabilities' to
   specify which operations to support, on which objects. This is the
diff --git a/packages/test_reflectable/test/substitute_http_test.dart b/packages/test_reflectable/test/substitute_http_test.dart
new file mode 100644
index 00000000..9464822f
--- /dev/null
+++ b/packages/test_reflectable/test/substitute_http_test.dart
@@ -0,0 +1,38 @@
+// Copyright (c) 2026, 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.
+
+// Test the special casing of declarations in 'dart:_http'.
+
+library test_reflectable.test.substitute_http_test;
+
+import 'dart:io';
+
+@GlobalQuantifyCapability(r'\.Cookie$', reflector)
+import 'package:reflectable/reflectable.dart';
+
+import 'substitute_http_test.reflectable.dart';
+
+class Reflector extends Reflectable {
+  const Reflector()
+    : super(
+        instanceInvokeCapability,
+        declarationsCapability,
+        superclassQuantifyCapability,
+        typeAnnotationQuantifyCapability,
+      );
+}
+
+const reflector = Reflector();
+
+@reflector
+class CookieHolder extends HttpException {
+  CookieHolder(super._);
+  final Cookie? cookie = null;
+}
+
+void main() {
+  initializeReflectable();
+  // Just checking that the program compiles, that is, it does not
+  // attempt to import `dart:_http`.
+}