diff --git a/pkg/front_end/lib/src/fasta/kernel/combined_member_signature.dart b/pkg/front_end/lib/src/fasta/kernel/combined_member_signature.dart
index f89b4e8..b1fcbcb 100644
--- a/pkg/front_end/lib/src/fasta/kernel/combined_member_signature.dart
+++ b/pkg/front_end/lib/src/fasta/kernel/combined_member_signature.dart
@@ -89,6 +89,8 @@
 
   bool _neededNnbdTopMerge = false;
 
+  bool _containsNnbdTypes = false;
+
   bool _needsCovarianceMerging = false;
 
   bool _isCombinedMemberSignatureCovarianceComputed = false;
@@ -222,6 +224,43 @@
     return _neededNnbdTopMerge;
   }
 
+  /// Returns `true` if the type of combined member signature has nnbd types.
+  ///
+  /// If the combined member signature for an opt-in class is computed from
+  /// identical legacy types, that is, without the need for nnbd top merge, then
+  /// the type will be copied over directly and a member created from the
+  /// combined member signature will therefore be a legacy member, even though
+  /// it is declared in an opt in class.
+  ///
+  /// To avoid reporting errors as if the member was an opt-in member, it is
+  /// marked as nullable-by-default.
+  ///
+  /// For instance
+  ///
+  ///    // opt out:
+  ///    mixin Mixin {
+  ///      void method({int named}) {}
+  ///    }
+  ///    // opt in:
+  ///    class Super {
+  ///      void method({required covariant int named}) {}
+  ///    }
+  ///    class Class extends Super with Mixin {
+  ///      // A forwarding stop for Mixin.method will be inserted here:
+  ///      // void method({covariant int named}) -> Mixin.method
+  ///    }
+  ///    class SubClass extends Class {
+  ///      // This is a valid override since `Class.method` should should
+  ///      // not be considered as _not_ having a required named parameter -
+  ///      // it is legacy and doesn't know about required named parameters.
+  ///      void method({required int named}) {}
+  ///    }
+  ///
+  bool get containsNnbdTypes {
+    _ensureCombinedMemberSignatureType();
+    return _containsNnbdTypes;
+  }
+
   /// Returns `true` if the covariance of the combined member signature is
   /// different from the covariance of the overridden member in the superclass.
   ///
@@ -283,6 +322,8 @@
       if (classBuilder.library.isNonNullableByDefault) {
         DartType canonicalMemberType =
             _combinedMemberSignatureType = getMemberType(_canonicalMemberIndex);
+        _containsNnbdTypes =
+            _getMember(_canonicalMemberIndex).isNonNullableByDefault;
         if (_mutualSubtypes != null) {
           _combinedMemberSignatureType =
               norm(_coreTypes, _combinedMemberSignatureType);
@@ -296,6 +337,7 @@
           }
           _neededNnbdTopMerge =
               canonicalMemberType != _combinedMemberSignatureType;
+          _containsNnbdTypes = _neededNnbdTopMerge;
         }
       } else {
         _combinedMemberSignatureType = getMemberType(_canonicalMemberIndex);
@@ -486,8 +528,7 @@
     )
       ..startFileOffset = startFileOffset
       ..fileOffset = fileOffset
-      ..isNonNullableByDefault =
-          enclosingClass.enclosingLibrary.isNonNullableByDefault
+      ..isNonNullableByDefault = containsNnbdTypes
       ..parent = enclosingClass;
   }
 
@@ -537,8 +578,7 @@
         reference: referenceFrom?.reference)
       ..startFileOffset = startFileOffset
       ..fileOffset = fileOffset
-      ..isNonNullableByDefault =
-          enclosingClass.enclosingLibrary.isNonNullableByDefault
+      ..isNonNullableByDefault = containsNnbdTypes
       ..parent = enclosingClass;
   }
 
@@ -617,8 +657,7 @@
         reference: referenceFrom?.reference)
       ..startFileOffset = startFileOffset
       ..fileOffset = fileOffset
-      ..isNonNullableByDefault =
-          enclosingClass.enclosingLibrary.isNonNullableByDefault
+      ..isNonNullableByDefault = containsNnbdTypes
       ..parent = enclosingClass;
   }
 
diff --git a/pkg/front_end/testcases/nnbd_mixed/flutter_issue_63029/flutter_issue_63029.dart.weak.expect b/pkg/front_end/testcases/nnbd_mixed/flutter_issue_63029/flutter_issue_63029.dart.weak.expect
index 2c462af..ed414cf 100644
--- a/pkg/front_end/testcases/nnbd_mixed/flutter_issue_63029/flutter_issue_63029.dart.weak.expect
+++ b/pkg/front_end/testcases/nnbd_mixed/flutter_issue_63029/flutter_issue_63029.dart.weak.expect
@@ -16,7 +16,7 @@
   synthetic constructor •() → self::_F&B&D
     : super flu::B::•()
     ;
-  abstract member-signature operator ==(dynamic other) → core::bool*; -> core::Object::==
+  abstract member-signature operator /*isNullableByDefault*/ ==(dynamic other) → core::bool*; -> core::Object::==
 }
 class F extends self::_F&B&D {
   synthetic constructor •() → self::F
diff --git a/pkg/front_end/testcases/nnbd_mixed/inheritance_from_opt_out.dart.weak.expect b/pkg/front_end/testcases/nnbd_mixed/inheritance_from_opt_out.dart.weak.expect
index 9ed16b6..dd9e9f0 100644
--- a/pkg/front_end/testcases/nnbd_mixed/inheritance_from_opt_out.dart.weak.expect
+++ b/pkg/front_end/testcases/nnbd_mixed/inheritance_from_opt_out.dart.weak.expect
@@ -34,13 +34,13 @@
   synthetic constructor •() → self::Class4b
     : super core::Object::•()
     ;
-  abstract member-signature operator ==(dynamic other) → core::bool*; -> core::Object::==
+  abstract member-signature operator /*isNullableByDefault*/ ==(dynamic other) → core::bool*; -> core::Object::==
 }
 class Class4c extends core::Object implements inh::GenericInterface<core::num?> {
   synthetic constructor •() → self::Class4c
     : super core::Object::•()
     ;
-  abstract member-signature operator ==(dynamic other) → core::bool*; -> core::Object::==
+  abstract member-signature operator /*isNullableByDefault*/ ==(dynamic other) → core::bool*; -> core::Object::==
 }
 class Class4d extends inh::LegacyClass4 implements inh::GenericInterface<core::num> {
   synthetic constructor •() → self::Class4d
diff --git a/pkg/front_end/testcases/nnbd_mixed/inheritance_from_opt_out.dart.weak.transformed.expect b/pkg/front_end/testcases/nnbd_mixed/inheritance_from_opt_out.dart.weak.transformed.expect
index 0bc6430..4b67f47 100644
--- a/pkg/front_end/testcases/nnbd_mixed/inheritance_from_opt_out.dart.weak.transformed.expect
+++ b/pkg/front_end/testcases/nnbd_mixed/inheritance_from_opt_out.dart.weak.transformed.expect
@@ -34,13 +34,13 @@
   synthetic constructor •() → self::Class4b
     : super core::Object::•()
     ;
-  abstract member-signature operator ==(dynamic other) → core::bool*; -> core::Object::==
+  abstract member-signature operator /*isNullableByDefault*/ ==(dynamic other) → core::bool*; -> core::Object::==
 }
 class Class4c extends core::Object implements inh::GenericInterface<core::num?> {
   synthetic constructor •() → self::Class4c
     : super core::Object::•()
     ;
-  abstract member-signature operator ==(dynamic other) → core::bool*; -> core::Object::==
+  abstract member-signature operator /*isNullableByDefault*/ ==(dynamic other) → core::bool*; -> core::Object::==
 }
 class Class4d extends inh::LegacyClass4 implements inh::GenericInterface<core::num> {
   synthetic constructor •() → self::Class4d
diff --git a/pkg/front_end/testcases/nnbd_mixed/mixin_from_opt_out.dart.weak.expect b/pkg/front_end/testcases/nnbd_mixed/mixin_from_opt_out.dart.weak.expect
index bd160a8..70ef7e5 100644
--- a/pkg/front_end/testcases/nnbd_mixed/mixin_from_opt_out.dart.weak.expect
+++ b/pkg/front_end/testcases/nnbd_mixed/mixin_from_opt_out.dart.weak.expect
@@ -9,7 +9,7 @@
   const synthetic constructor •() → self::_Class&Object&Mixin
     : super core::Object::•()
     ;
-  abstract member-signature operator ==(dynamic other) → core::bool*; -> core::Object::==
+  abstract member-signature operator /*isNullableByDefault*/ ==(dynamic other) → core::bool*; -> core::Object::==
 }
 class Class extends self::_Class&Object&Mixin {
   synthetic constructor •() → self::Class
diff --git a/pkg/front_end/testcases/nnbd_mixed/required_name_override.dart.weak.expect b/pkg/front_end/testcases/nnbd_mixed/required_name_override.dart.weak.expect
index e2f0b61..310bd85 100644
--- a/pkg/front_end/testcases/nnbd_mixed/required_name_override.dart.weak.expect
+++ b/pkg/front_end/testcases/nnbd_mixed/required_name_override.dart.weak.expect
@@ -36,7 +36,7 @@
   method test_default({required core::int? i = #C1}) → void {}
   method test_nondefault({required core::int? i = #C1}) → void {}
   method test_legacy({required core::int? i = #C1}) → void {}
-  abstract member-signature operator ==(dynamic other) → core::bool*; -> core::Object::==
+  abstract member-signature operator /*isNullableByDefault*/ ==(dynamic other) → core::bool*; -> core::Object::==
 }
 static method main() → dynamic {
   new self::A::•().{self::A::test_default}(i: 1);
diff --git a/pkg/front_end/testcases/nnbd_mixed/required_name_override.dart.weak.transformed.expect b/pkg/front_end/testcases/nnbd_mixed/required_name_override.dart.weak.transformed.expect
index e2f0b61..310bd85 100644
--- a/pkg/front_end/testcases/nnbd_mixed/required_name_override.dart.weak.transformed.expect
+++ b/pkg/front_end/testcases/nnbd_mixed/required_name_override.dart.weak.transformed.expect
@@ -36,7 +36,7 @@
   method test_default({required core::int? i = #C1}) → void {}
   method test_nondefault({required core::int? i = #C1}) → void {}
   method test_legacy({required core::int? i = #C1}) → void {}
-  abstract member-signature operator ==(dynamic other) → core::bool*; -> core::Object::==
+  abstract member-signature operator /*isNullableByDefault*/ ==(dynamic other) → core::bool*; -> core::Object::==
 }
 static method main() → dynamic {
   new self::A::•().{self::A::test_default}(i: 1);
diff --git a/pkg/front_end/testcases/nnbd_mixed/required_parameter_mixed_from_opt_out.dart b/pkg/front_end/testcases/nnbd_mixed/required_parameter_mixed_from_opt_out.dart
new file mode 100644
index 0000000..a1abf5e
--- /dev/null
+++ b/pkg/front_end/testcases/nnbd_mixed/required_parameter_mixed_from_opt_out.dart
@@ -0,0 +1,17 @@
+// Copyright (c) 2020, 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.
+
+import 'required_parameter_mixed_from_opt_out_lib.dart';
+
+class Super {
+  void method({required covariant int named}) {}
+}
+
+class Class extends Super with Mixin {}
+
+class SubClass extends Class {
+  void method({required covariant int named}) {}
+}
+
+main() {}
diff --git a/pkg/front_end/testcases/nnbd_mixed/required_parameter_mixed_from_opt_out.dart.textual_outline.expect b/pkg/front_end/testcases/nnbd_mixed/required_parameter_mixed_from_opt_out.dart.textual_outline.expect
new file mode 100644
index 0000000..ef719ee
--- /dev/null
+++ b/pkg/front_end/testcases/nnbd_mixed/required_parameter_mixed_from_opt_out.dart.textual_outline.expect
@@ -0,0 +1,13 @@
+import 'required_parameter_mixed_from_opt_out_lib.dart';
+
+class Super {
+  void method({required covariant int named}) {}
+}
+
+class Class extends Super with Mixin {}
+
+class SubClass extends Class {
+  void method({required covariant int named}) {}
+}
+
+main() {}
diff --git a/pkg/front_end/testcases/nnbd_mixed/required_parameter_mixed_from_opt_out.dart.textual_outline_modelled.expect b/pkg/front_end/testcases/nnbd_mixed/required_parameter_mixed_from_opt_out.dart.textual_outline_modelled.expect
new file mode 100644
index 0000000..45cf3b0
--- /dev/null
+++ b/pkg/front_end/testcases/nnbd_mixed/required_parameter_mixed_from_opt_out.dart.textual_outline_modelled.expect
@@ -0,0 +1,13 @@
+import 'required_parameter_mixed_from_opt_out_lib.dart';
+
+class Class extends Super with Mixin {}
+
+class SubClass extends Class {
+  void method({required covariant int named}) {}
+}
+
+class Super {
+  void method({required covariant int named}) {}
+}
+
+main() {}
diff --git a/pkg/front_end/testcases/nnbd_mixed/required_parameter_mixed_from_opt_out.dart.weak.expect b/pkg/front_end/testcases/nnbd_mixed/required_parameter_mixed_from_opt_out.dart.weak.expect
new file mode 100644
index 0000000..8c936d9
--- /dev/null
+++ b/pkg/front_end/testcases/nnbd_mixed/required_parameter_mixed_from_opt_out.dart.weak.expect
@@ -0,0 +1,55 @@
+library /*isNonNullableByDefault*/;
+import self as self;
+import "dart:core" as core;
+import "required_parameter_mixed_from_opt_out_lib.dart" as req;
+
+import "org-dartlang-testcase:///required_parameter_mixed_from_opt_out_lib.dart";
+
+class Super extends core::Object {
+  synthetic constructor •() → self::Super
+    : super core::Object::•()
+    ;
+  method method({required covariant core::int named = #C1}) → void {}
+}
+abstract class _Class&Super&Mixin = self::Super with req::Mixin /*isAnonymousMixin*/  {
+  synthetic constructor •() → self::_Class&Super&Mixin
+    : super self::Super::•()
+    ;
+  forwarding-stub method /*isNullableByDefault*/ method({covariant core::int* named = #C1}) → void
+    return super.{self::Super::method}(named: named);
+  abstract member-signature operator /*isNullableByDefault*/ ==(dynamic other) → core::bool*; -> core::Object::==
+}
+class Class extends self::_Class&Super&Mixin {
+  synthetic constructor •() → self::Class
+    : super self::_Class&Super&Mixin::•()
+    ;
+}
+class SubClass extends self::Class {
+  synthetic constructor •() → self::SubClass
+    : super self::Class::•()
+    ;
+  method method({required covariant core::int named = #C1}) → void {}
+}
+static method main() → dynamic {}
+
+library;
+import self as req;
+import "dart:core" as core;
+
+abstract class Mixin extends core::Object /*isMixinDeclaration*/  {
+  method method({core::int* named = #C1}) → void {}
+  abstract member-signature get _identityHashCode() → core::int*; -> core::Object::_identityHashCode
+  abstract member-signature method _instanceOf(dynamic instantiatorTypeArguments, dynamic functionTypeArguments, dynamic type) → core::bool*; -> core::Object::_instanceOf
+  abstract member-signature method _simpleInstanceOf(dynamic type) → core::bool*; -> core::Object::_simpleInstanceOf
+  abstract member-signature method _simpleInstanceOfTrue(dynamic type) → core::bool*; -> core::Object::_simpleInstanceOfTrue
+  abstract member-signature method _simpleInstanceOfFalse(dynamic type) → core::bool*; -> core::Object::_simpleInstanceOfFalse
+  abstract member-signature operator ==(dynamic other) → core::bool*; -> core::Object::==
+  abstract member-signature get hashCode() → core::int*; -> core::Object::hashCode
+  abstract member-signature method toString() → core::String*; -> core::Object::toString
+  abstract member-signature method noSuchMethod(core::Invocation* invocation) → dynamic; -> core::Object::noSuchMethod
+  abstract member-signature get runtimeType() → core::Type*; -> core::Object::runtimeType
+}
+
+constants  {
+  #C1 = null
+}
diff --git a/pkg/front_end/testcases/nnbd_mixed/required_parameter_mixed_from_opt_out.dart.weak.transformed.expect b/pkg/front_end/testcases/nnbd_mixed/required_parameter_mixed_from_opt_out.dart.weak.transformed.expect
new file mode 100644
index 0000000..5db65ce
--- /dev/null
+++ b/pkg/front_end/testcases/nnbd_mixed/required_parameter_mixed_from_opt_out.dart.weak.transformed.expect
@@ -0,0 +1,63 @@
+library /*isNonNullableByDefault*/;
+import self as self;
+import "dart:core" as core;
+import "required_parameter_mixed_from_opt_out_lib.dart" as req;
+
+import "org-dartlang-testcase:///required_parameter_mixed_from_opt_out_lib.dart";
+
+class Super extends core::Object {
+  synthetic constructor •() → self::Super
+    : super core::Object::•()
+    ;
+  method method({required covariant core::int named = #C1}) → void {}
+}
+abstract class _Class&Super&Mixin extends self::Super implements req::Mixin /*isAnonymousMixin,isEliminatedMixin*/  {
+  synthetic constructor •() → self::_Class&Super&Mixin
+    : super self::Super::•()
+    ;
+  method /*isNullableByDefault, from org-dartlang-testcase:///required_parameter_mixed_from_opt_out_lib.dart */ method({covariant core::int* named = #C1}) → void {}
+  abstract member-signature operator /*isNullableByDefault, from org-dartlang-testcase:///required_parameter_mixed_from_opt_out_lib.dart */ ==(dynamic other) → core::bool*; -> core::Object::==
+  abstract member-signature get /*isNullableByDefault, from org-dartlang-testcase:///required_parameter_mixed_from_opt_out_lib.dart */ _identityHashCode() → core::int*; -> core::Object::_identityHashCode
+  abstract member-signature method /*isNullableByDefault, from org-dartlang-testcase:///required_parameter_mixed_from_opt_out_lib.dart */ _instanceOf(dynamic instantiatorTypeArguments, dynamic functionTypeArguments, dynamic type) → core::bool*; -> core::Object::_instanceOf
+  abstract member-signature method /*isNullableByDefault, from org-dartlang-testcase:///required_parameter_mixed_from_opt_out_lib.dart */ _simpleInstanceOf(dynamic type) → core::bool*; -> core::Object::_simpleInstanceOf
+  abstract member-signature method /*isNullableByDefault, from org-dartlang-testcase:///required_parameter_mixed_from_opt_out_lib.dart */ _simpleInstanceOfTrue(dynamic type) → core::bool*; -> core::Object::_simpleInstanceOfTrue
+  abstract member-signature method /*isNullableByDefault, from org-dartlang-testcase:///required_parameter_mixed_from_opt_out_lib.dart */ _simpleInstanceOfFalse(dynamic type) → core::bool*; -> core::Object::_simpleInstanceOfFalse
+  abstract member-signature get /*isNullableByDefault, from org-dartlang-testcase:///required_parameter_mixed_from_opt_out_lib.dart */ hashCode() → core::int*; -> core::Object::hashCode
+  abstract member-signature method /*isNullableByDefault, from org-dartlang-testcase:///required_parameter_mixed_from_opt_out_lib.dart */ toString() → core::String*; -> core::Object::toString
+  abstract member-signature method /*isNullableByDefault, from org-dartlang-testcase:///required_parameter_mixed_from_opt_out_lib.dart */ noSuchMethod(core::Invocation* invocation) → dynamic; -> core::Object::noSuchMethod
+  abstract member-signature get /*isNullableByDefault, from org-dartlang-testcase:///required_parameter_mixed_from_opt_out_lib.dart */ runtimeType() → core::Type*; -> core::Object::runtimeType
+}
+class Class extends self::_Class&Super&Mixin {
+  synthetic constructor •() → self::Class
+    : super self::_Class&Super&Mixin::•()
+    ;
+}
+class SubClass extends self::Class {
+  synthetic constructor •() → self::SubClass
+    : super self::Class::•()
+    ;
+  method method({required covariant core::int named = #C1}) → void {}
+}
+static method main() → dynamic {}
+
+library;
+import self as req;
+import "dart:core" as core;
+
+abstract class Mixin extends core::Object /*isMixinDeclaration*/  {
+  method method({core::int* named = #C1}) → void {}
+  abstract member-signature get _identityHashCode() → core::int*; -> core::Object::_identityHashCode
+  abstract member-signature method _instanceOf(dynamic instantiatorTypeArguments, dynamic functionTypeArguments, dynamic type) → core::bool*; -> core::Object::_instanceOf
+  abstract member-signature method _simpleInstanceOf(dynamic type) → core::bool*; -> core::Object::_simpleInstanceOf
+  abstract member-signature method _simpleInstanceOfTrue(dynamic type) → core::bool*; -> core::Object::_simpleInstanceOfTrue
+  abstract member-signature method _simpleInstanceOfFalse(dynamic type) → core::bool*; -> core::Object::_simpleInstanceOfFalse
+  abstract member-signature operator ==(dynamic other) → core::bool*; -> core::Object::==
+  abstract member-signature get hashCode() → core::int*; -> core::Object::hashCode
+  abstract member-signature method toString() → core::String*; -> core::Object::toString
+  abstract member-signature method noSuchMethod(core::Invocation* invocation) → dynamic; -> core::Object::noSuchMethod
+  abstract member-signature get runtimeType() → core::Type*; -> core::Object::runtimeType
+}
+
+constants  {
+  #C1 = null
+}
diff --git a/pkg/front_end/testcases/nnbd_mixed/required_parameter_mixed_from_opt_out_lib.dart b/pkg/front_end/testcases/nnbd_mixed/required_parameter_mixed_from_opt_out_lib.dart
new file mode 100644
index 0000000..922c203
--- /dev/null
+++ b/pkg/front_end/testcases/nnbd_mixed/required_parameter_mixed_from_opt_out_lib.dart
@@ -0,0 +1,9 @@
+// Copyright (c) 2020, 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.
+
+// @dart=2.9
+
+mixin Mixin {
+  void method({int named}) {}
+}
diff --git a/tools/VERSION b/tools/VERSION
index b27aeb5..cabd085 100644
--- a/tools/VERSION
+++ b/tools/VERSION
@@ -27,5 +27,5 @@
 MAJOR 2
 MINOR 12
 PATCH 0
-PRERELEASE 85
+PRERELEASE 86
 PRERELEASE_PATCH 0
\ No newline at end of file