diff --git a/pkg/front_end/lib/src/fasta/builder/class_builder.dart b/pkg/front_end/lib/src/fasta/builder/class_builder.dart
index 7a7f258..8aeafe2 100644
--- a/pkg/front_end/lib/src/fasta/builder/class_builder.dart
+++ b/pkg/front_end/lib/src/fasta/builder/class_builder.dart
@@ -1416,6 +1416,7 @@
 
   @override
   void checkMixinApplication(ClassHierarchy hierarchy, CoreTypes coreTypes) {
+    TypeEnvironment typeEnvironment = new TypeEnvironment(coreTypes, hierarchy);
     // A mixin declaration can only be applied to a class that implements all
     // the declaration's superclass constraints.
     InterfaceType supertype = cls.supertype.asInterfaceType;
@@ -1425,7 +1426,13 @@
           substitution.substituteSupertype(constraint).asInterfaceType;
       InterfaceType implementedInterface = hierarchy.getTypeAsInstanceOf(
           supertype, requiredInterface.classNode, library.library, coreTypes);
-      if (implementedInterface != requiredInterface) {
+      if (implementedInterface == null ||
+          !typeEnvironment.areMutualSubtypes(
+              implementedInterface,
+              requiredInterface,
+              library.isNonNullableByDefault
+                  ? SubtypeCheckMode.withNullabilities
+                  : SubtypeCheckMode.ignoringNullabilities)) {
         library.addProblem(
             templateMixinApplicationIncompatibleSupertype.withArguments(
                 supertype,
diff --git a/pkg/front_end/test/spell_checking_list_tests.txt b/pkg/front_end/test/spell_checking_list_tests.txt
index cad7625..64c9d24 100644
--- a/pkg/front_end/test/spell_checking_list_tests.txt
+++ b/pkg/front_end/test/spell_checking_list_tests.txt
@@ -75,12 +75,14 @@
 bulk2
 bulkcompile
 c's
+ca
 cafebabe
 callable
 camel
 capitalized
 casing
 causal
+cb
 cc
 ccc
 charcode
@@ -89,6 +91,17 @@
 checkout
 cipd
 circular
+class1a
+class1b
+class2c
+class2d
+class2e
+class2f
+class3d
+class5a
+class5b
+class5c
+class5d
 cli
 cloneable
 cmp
diff --git a/pkg/front_end/testcases/nnbd_mixed/bad_mixins.dart b/pkg/front_end/testcases/nnbd_mixed/bad_mixins.dart
new file mode 100644
index 0000000..49e99ca
--- /dev/null
+++ b/pkg/front_end/testcases/nnbd_mixed/bad_mixins.dart
@@ -0,0 +1,35 @@
+// 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=2.7
+
+import 'bad_mixins_lib.dart';
+
+class Class1a extends Object with MixinA {} // error
+
+class Class1b extends A with MixinA {} // ok
+
+class Class2a extends Object with MixinB<int> {} // error
+
+class Class2c extends B<num> with MixinB<int> {} // error
+
+class Class2d extends B<int> with MixinB<int> {} // ok
+
+class Class2e extends B<Object> with MixinB<dynamic> {} // error
+
+class Class3a extends Object with MixinC<num, int> {} // error
+
+class Class3b extends C<num, num> with MixinC<num, int> {} // error
+
+class Class3d extends C<num, int> with MixinC<num, int> {} // ok
+
+class Class4a extends ClassBa with MixinB<int> {} // ok
+
+class Class4b extends ClassBb with MixinB<int> {} // ok
+
+class Class5a extends ClassCa with MixinC<num, int> {} // ok
+
+class Class5b extends ClassCb with MixinC<num, int> {} // ok
+
+main() {}
diff --git a/pkg/front_end/testcases/nnbd_mixed/bad_mixins.dart.textual_outline.expect b/pkg/front_end/testcases/nnbd_mixed/bad_mixins.dart.textual_outline.expect
new file mode 100644
index 0000000..f9ea90e
--- /dev/null
+++ b/pkg/front_end/testcases/nnbd_mixed/bad_mixins.dart.textual_outline.expect
@@ -0,0 +1,30 @@
+// @dart = 2.7
+import 'bad_mixins_lib.dart';
+
+class Class1a extends Object with MixinA {}
+
+class Class1b extends A with MixinA {}
+
+class Class2a extends Object with MixinB<int> {}
+
+class Class2c extends B<num> with MixinB<int> {}
+
+class Class2d extends B<int> with MixinB<int> {}
+
+class Class2e extends B<Object> with MixinB<dynamic> {}
+
+class Class3a extends Object with MixinC<num, int> {}
+
+class Class3b extends C<num, num> with MixinC<num, int> {}
+
+class Class3d extends C<num, int> with MixinC<num, int> {}
+
+class Class4a extends ClassBa with MixinB<int> {}
+
+class Class4b extends ClassBb with MixinB<int> {}
+
+class Class5a extends ClassCa with MixinC<num, int> {}
+
+class Class5b extends ClassCb with MixinC<num, int> {}
+
+main() {}
diff --git a/pkg/front_end/testcases/nnbd_mixed/bad_mixins.dart.textual_outline_modelled.expect b/pkg/front_end/testcases/nnbd_mixed/bad_mixins.dart.textual_outline_modelled.expect
new file mode 100644
index 0000000..f9ea90e
--- /dev/null
+++ b/pkg/front_end/testcases/nnbd_mixed/bad_mixins.dart.textual_outline_modelled.expect
@@ -0,0 +1,30 @@
+// @dart = 2.7
+import 'bad_mixins_lib.dart';
+
+class Class1a extends Object with MixinA {}
+
+class Class1b extends A with MixinA {}
+
+class Class2a extends Object with MixinB<int> {}
+
+class Class2c extends B<num> with MixinB<int> {}
+
+class Class2d extends B<int> with MixinB<int> {}
+
+class Class2e extends B<Object> with MixinB<dynamic> {}
+
+class Class3a extends Object with MixinC<num, int> {}
+
+class Class3b extends C<num, num> with MixinC<num, int> {}
+
+class Class3d extends C<num, int> with MixinC<num, int> {}
+
+class Class4a extends ClassBa with MixinB<int> {}
+
+class Class4b extends ClassBb with MixinB<int> {}
+
+class Class5a extends ClassCa with MixinC<num, int> {}
+
+class Class5b extends ClassCb with MixinC<num, int> {}
+
+main() {}
diff --git a/pkg/front_end/testcases/nnbd_mixed/bad_mixins.dart.weak.expect b/pkg/front_end/testcases/nnbd_mixed/bad_mixins.dart.weak.expect
new file mode 100644
index 0000000..655c8ca
--- /dev/null
+++ b/pkg/front_end/testcases/nnbd_mixed/bad_mixins.dart.weak.expect
@@ -0,0 +1,698 @@
+//
+// Problems in component:
+//
+// pkg/front_end/testcases/nnbd_mixed/bad_mixins.dart:15:7: Error: 'B with MixinB' can't implement both 'B<num>' and 'B<int>'
+//  - 'B' is from 'pkg/front_end/testcases/nnbd_mixed/bad_mixins_lib.dart'.
+// class Class2c extends B<num> with MixinB<int> {} // error
+//       ^
+//
+// pkg/front_end/testcases/nnbd_mixed/bad_mixins.dart:19:7: Error: 'B with MixinB' can't implement both 'B<Object>' and 'B<dynamic>'
+//  - 'B' is from 'pkg/front_end/testcases/nnbd_mixed/bad_mixins_lib.dart'.
+//  - 'Object' is from 'dart:core'.
+// class Class2e extends B<Object> with MixinB<dynamic> {} // error
+//       ^
+//
+// pkg/front_end/testcases/nnbd_mixed/bad_mixins.dart:23:7: Error: 'C with MixinC' can't implement both 'C<num, num>' and 'C<num, int>'
+//  - 'C' is from 'pkg/front_end/testcases/nnbd_mixed/bad_mixins_lib.dart'.
+// class Class3b extends C<num, num> with MixinC<num, int> {} // error
+//       ^
+//
+// pkg/front_end/testcases/nnbd_mixed/bad_mixins_lib.dart:23:7: Error: 'B with MixinB' can't implement both 'B<int?>' and 'B<int>'
+//  - 'B' is from 'pkg/front_end/testcases/nnbd_mixed/bad_mixins_lib.dart'.
+// class Class2b extends B<int?> with MixinB<int> {} // error
+//       ^
+//
+// pkg/front_end/testcases/nnbd_mixed/bad_mixins_lib.dart:25:7: Error: 'B with MixinB' can't implement both 'B<num>' and 'B<int>'
+//  - 'B' is from 'pkg/front_end/testcases/nnbd_mixed/bad_mixins_lib.dart'.
+// class Class2c extends B<num> with MixinB<int> {} // error
+//       ^
+//
+// pkg/front_end/testcases/nnbd_mixed/bad_mixins_lib.dart:29:7: Error: 'B with MixinB' can't implement both 'B<Object>' and 'B<dynamic>'
+//  - 'B' is from 'pkg/front_end/testcases/nnbd_mixed/bad_mixins_lib.dart'.
+//  - 'Object' is from 'dart:core'.
+// class Class2e extends B<Object> with MixinB<dynamic> {} // error
+//       ^
+//
+// pkg/front_end/testcases/nnbd_mixed/bad_mixins_lib.dart:35:7: Error: 'C with MixinC' can't implement both 'C<num, num>' and 'C<num, int>'
+//  - 'C' is from 'pkg/front_end/testcases/nnbd_mixed/bad_mixins_lib.dart'.
+// class Class3b extends C<num, num> with MixinC<num, int> {} // error
+//       ^
+//
+// pkg/front_end/testcases/nnbd_mixed/bad_mixins_lib.dart:37:7: Error: 'C with MixinC' can't implement both 'C<num?, int>' and 'C<num, int>'
+//  - 'C' is from 'pkg/front_end/testcases/nnbd_mixed/bad_mixins_lib.dart'.
+// class Class3c extends C<num?, int> with MixinC<num, int> {} // error
+//       ^
+//
+// pkg/front_end/testcases/nnbd_mixed/bad_mixins_lib.dart:49:7: Error: 'ClassBa with MixinB' can't implement both 'B<int?>' and 'B<int>'
+//  - 'B' is from 'pkg/front_end/testcases/nnbd_mixed/bad_mixins_lib.dart'.
+// class Class4a extends ClassBa with MixinB<int> {} // error
+//       ^
+//
+// pkg/front_end/testcases/nnbd_mixed/bad_mixins_lib.dart:53:7: Error: 'ClassBb with MixinB' can't implement both 'B<int>' and 'B<int?>'
+//  - 'B' is from 'pkg/front_end/testcases/nnbd_mixed/bad_mixins_lib.dart'.
+// class Class4c extends ClassBb with MixinB<int?> {} // error
+//       ^
+//
+// pkg/front_end/testcases/nnbd_mixed/bad_mixins_lib.dart:57:7: Error: 'ClassCa with MixinC' can't implement both 'C<num?, int?>' and 'C<num?, int>'
+//  - 'C' is from 'pkg/front_end/testcases/nnbd_mixed/bad_mixins_lib.dart'.
+// class Class5a extends ClassCa with MixinC<num?, int> {} // error
+//       ^
+//
+// pkg/front_end/testcases/nnbd_mixed/bad_mixins_lib.dart:61:7: Error: 'ClassCb with MixinC' can't implement both 'C<num?, int>' and 'C<num?, int?>'
+//  - 'C' is from 'pkg/front_end/testcases/nnbd_mixed/bad_mixins_lib.dart'.
+// class Class5c extends ClassCb with MixinC<num?, int?> {} // error
+//       ^
+//
+library;
+//
+// Problems in library:
+//
+// pkg/front_end/testcases/nnbd_mixed/bad_mixins.dart:9:7: Error: 'Object' doesn't implement 'A' so it can't be used with 'MixinA'.
+//  - 'Object' is from 'dart:core'.
+//  - 'A' is from 'pkg/front_end/testcases/nnbd_mixed/bad_mixins_lib.dart'.
+//  - 'MixinA' is from 'pkg/front_end/testcases/nnbd_mixed/bad_mixins_lib.dart'.
+// class Class1a extends Object with MixinA {} // error
+//       ^
+//
+// pkg/front_end/testcases/nnbd_mixed/bad_mixins.dart:13:7: Error: 'Object' doesn't implement 'B<int>' so it can't be used with 'MixinB<int>'.
+//  - 'Object' is from 'dart:core'.
+//  - 'B' is from 'pkg/front_end/testcases/nnbd_mixed/bad_mixins_lib.dart'.
+//  - 'MixinB' is from 'pkg/front_end/testcases/nnbd_mixed/bad_mixins_lib.dart'.
+// class Class2a extends Object with MixinB<int> {} // error
+//       ^
+//
+// pkg/front_end/testcases/nnbd_mixed/bad_mixins.dart:15:7: Error: 'B<num>' doesn't implement 'B<int>' so it can't be used with 'MixinB<int>'.
+//  - 'B' is from 'pkg/front_end/testcases/nnbd_mixed/bad_mixins_lib.dart'.
+//  - 'MixinB' is from 'pkg/front_end/testcases/nnbd_mixed/bad_mixins_lib.dart'.
+// class Class2c extends B<num> with MixinB<int> {} // error
+//       ^
+//
+// pkg/front_end/testcases/nnbd_mixed/bad_mixins.dart:21:7: Error: 'Object' doesn't implement 'C<num, int>' so it can't be used with 'MixinC<num, int>'.
+//  - 'Object' is from 'dart:core'.
+//  - 'C' is from 'pkg/front_end/testcases/nnbd_mixed/bad_mixins_lib.dart'.
+//  - 'MixinC' is from 'pkg/front_end/testcases/nnbd_mixed/bad_mixins_lib.dart'.
+// class Class3a extends Object with MixinC<num, int> {} // error
+//       ^
+//
+// pkg/front_end/testcases/nnbd_mixed/bad_mixins.dart:23:7: Error: 'C<num, num>' doesn't implement 'C<num, int>' so it can't be used with 'MixinC<num, int>'.
+//  - 'C' is from 'pkg/front_end/testcases/nnbd_mixed/bad_mixins_lib.dart'.
+//  - 'MixinC' is from 'pkg/front_end/testcases/nnbd_mixed/bad_mixins_lib.dart'.
+// class Class3b extends C<num, num> with MixinC<num, int> {} // error
+//       ^
+//
+import self as self;
+import "dart:core" as core;
+import "bad_mixins_lib.dart" as bad;
+
+import "org-dartlang-testcase:///bad_mixins_lib.dart";
+
+abstract class _Class1a&Object&MixinA = core::Object with bad::MixinA /*isAnonymousMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_Class1a&Object&MixinA*
+    : super core::Object::•()
+    ;
+  abstract member-signature get _identityHashCode() → core::int*;
+  abstract member-signature method _instanceOf(dynamic instantiatorTypeArguments, dynamic functionTypeArguments, dynamic type) → core::bool*;
+  abstract member-signature method _simpleInstanceOf(dynamic type) → core::bool*;
+  abstract member-signature method _simpleInstanceOfTrue(dynamic type) → core::bool*;
+  abstract member-signature method _simpleInstanceOfFalse(dynamic type) → core::bool*;
+  abstract member-signature operator ==(dynamic other) → core::bool*;
+  abstract member-signature get hashCode() → core::int*;
+  abstract member-signature method toString() → core::String*;
+  abstract member-signature method noSuchMethod(core::Invocation* invocation) → dynamic;
+  abstract member-signature get runtimeType() → core::Type*;
+}
+class Class1a extends self::_Class1a&Object&MixinA {
+  synthetic constructor •() → self::Class1a*
+    : super self::_Class1a&Object&MixinA::•()
+    ;
+}
+abstract class _Class1b&A&MixinA = bad::A with bad::MixinA /*isAnonymousMixin*/  {
+  synthetic constructor •() → self::_Class1b&A&MixinA*
+    : super bad::A::•()
+    ;
+  abstract member-signature get _identityHashCode() → core::int*;
+  abstract member-signature method _instanceOf(dynamic instantiatorTypeArguments, dynamic functionTypeArguments, dynamic type) → core::bool*;
+  abstract member-signature method _simpleInstanceOf(dynamic type) → core::bool*;
+  abstract member-signature method _simpleInstanceOfTrue(dynamic type) → core::bool*;
+  abstract member-signature method _simpleInstanceOfFalse(dynamic type) → core::bool*;
+  abstract member-signature operator ==(dynamic other) → core::bool*;
+  abstract member-signature get hashCode() → core::int*;
+  abstract member-signature method toString() → core::String*;
+  abstract member-signature method noSuchMethod(core::Invocation* invocation) → dynamic;
+  abstract member-signature get runtimeType() → core::Type*;
+}
+class Class1b extends self::_Class1b&A&MixinA {
+  synthetic constructor •() → self::Class1b*
+    : super self::_Class1b&A&MixinA::•()
+    ;
+}
+abstract class _Class2a&Object&MixinB = core::Object with bad::MixinB<core::int*> /*isAnonymousMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_Class2a&Object&MixinB*
+    : super core::Object::•()
+    ;
+  abstract member-signature get _identityHashCode() → core::int*;
+  abstract member-signature method _instanceOf(dynamic instantiatorTypeArguments, dynamic functionTypeArguments, dynamic type) → core::bool*;
+  abstract member-signature method _simpleInstanceOf(dynamic type) → core::bool*;
+  abstract member-signature method _simpleInstanceOfTrue(dynamic type) → core::bool*;
+  abstract member-signature method _simpleInstanceOfFalse(dynamic type) → core::bool*;
+  abstract member-signature operator ==(dynamic other) → core::bool*;
+  abstract member-signature get hashCode() → core::int*;
+  abstract member-signature method toString() → core::String*;
+  abstract member-signature method noSuchMethod(core::Invocation* invocation) → dynamic;
+  abstract member-signature get runtimeType() → core::Type*;
+}
+class Class2a extends self::_Class2a&Object&MixinB {
+  synthetic constructor •() → self::Class2a*
+    : super self::_Class2a&Object&MixinB::•()
+    ;
+}
+abstract class _Class2c&B&MixinB = bad::B<core::num*> with bad::MixinB<core::int*> /*isAnonymousMixin*/  {
+  synthetic constructor •() → self::_Class2c&B&MixinB*
+    : super bad::B::•()
+    ;
+  abstract member-signature get _identityHashCode() → core::int*;
+  abstract member-signature method _instanceOf(dynamic instantiatorTypeArguments, dynamic functionTypeArguments, dynamic type) → core::bool*;
+  abstract member-signature method _simpleInstanceOf(dynamic type) → core::bool*;
+  abstract member-signature method _simpleInstanceOfTrue(dynamic type) → core::bool*;
+  abstract member-signature method _simpleInstanceOfFalse(dynamic type) → core::bool*;
+  abstract member-signature operator ==(dynamic other) → core::bool*;
+  abstract member-signature get hashCode() → core::int*;
+  abstract member-signature method toString() → core::String*;
+  abstract member-signature method noSuchMethod(core::Invocation* invocation) → dynamic;
+  abstract member-signature get runtimeType() → core::Type*;
+}
+class Class2c extends self::_Class2c&B&MixinB {
+  synthetic constructor •() → self::Class2c*
+    : super self::_Class2c&B&MixinB::•()
+    ;
+}
+abstract class _Class2d&B&MixinB = bad::B<core::int*> with bad::MixinB<core::int*> /*isAnonymousMixin*/  {
+  synthetic constructor •() → self::_Class2d&B&MixinB*
+    : super bad::B::•()
+    ;
+  abstract member-signature get _identityHashCode() → core::int*;
+  abstract member-signature method _instanceOf(dynamic instantiatorTypeArguments, dynamic functionTypeArguments, dynamic type) → core::bool*;
+  abstract member-signature method _simpleInstanceOf(dynamic type) → core::bool*;
+  abstract member-signature method _simpleInstanceOfTrue(dynamic type) → core::bool*;
+  abstract member-signature method _simpleInstanceOfFalse(dynamic type) → core::bool*;
+  abstract member-signature operator ==(dynamic other) → core::bool*;
+  abstract member-signature get hashCode() → core::int*;
+  abstract member-signature method toString() → core::String*;
+  abstract member-signature method noSuchMethod(core::Invocation* invocation) → dynamic;
+  abstract member-signature get runtimeType() → core::Type*;
+}
+class Class2d extends self::_Class2d&B&MixinB {
+  synthetic constructor •() → self::Class2d*
+    : super self::_Class2d&B&MixinB::•()
+    ;
+}
+abstract class _Class2e&B&MixinB = bad::B<core::Object*> with bad::MixinB<dynamic> /*isAnonymousMixin*/  {
+  synthetic constructor •() → self::_Class2e&B&MixinB*
+    : super bad::B::•()
+    ;
+  abstract member-signature get _identityHashCode() → core::int*;
+  abstract member-signature method _instanceOf(dynamic instantiatorTypeArguments, dynamic functionTypeArguments, dynamic type) → core::bool*;
+  abstract member-signature method _simpleInstanceOf(dynamic type) → core::bool*;
+  abstract member-signature method _simpleInstanceOfTrue(dynamic type) → core::bool*;
+  abstract member-signature method _simpleInstanceOfFalse(dynamic type) → core::bool*;
+  abstract member-signature operator ==(dynamic other) → core::bool*;
+  abstract member-signature get hashCode() → core::int*;
+  abstract member-signature method toString() → core::String*;
+  abstract member-signature method noSuchMethod(core::Invocation* invocation) → dynamic;
+  abstract member-signature get runtimeType() → core::Type*;
+}
+class Class2e extends self::_Class2e&B&MixinB {
+  synthetic constructor •() → self::Class2e*
+    : super self::_Class2e&B&MixinB::•()
+    ;
+}
+abstract class _Class3a&Object&MixinC = core::Object with bad::MixinC<core::num*, core::int*> /*isAnonymousMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_Class3a&Object&MixinC*
+    : super core::Object::•()
+    ;
+  abstract member-signature get _identityHashCode() → core::int*;
+  abstract member-signature method _instanceOf(dynamic instantiatorTypeArguments, dynamic functionTypeArguments, dynamic type) → core::bool*;
+  abstract member-signature method _simpleInstanceOf(dynamic type) → core::bool*;
+  abstract member-signature method _simpleInstanceOfTrue(dynamic type) → core::bool*;
+  abstract member-signature method _simpleInstanceOfFalse(dynamic type) → core::bool*;
+  abstract member-signature operator ==(dynamic other) → core::bool*;
+  abstract member-signature get hashCode() → core::int*;
+  abstract member-signature method toString() → core::String*;
+  abstract member-signature method noSuchMethod(core::Invocation* invocation) → dynamic;
+  abstract member-signature get runtimeType() → core::Type*;
+}
+class Class3a extends self::_Class3a&Object&MixinC {
+  synthetic constructor •() → self::Class3a*
+    : super self::_Class3a&Object&MixinC::•()
+    ;
+}
+abstract class _Class3b&C&MixinC = bad::C<core::num*, core::num*> with bad::MixinC<core::num*, core::int*> /*isAnonymousMixin*/  {
+  synthetic constructor •() → self::_Class3b&C&MixinC*
+    : super bad::C::•()
+    ;
+  abstract member-signature get _identityHashCode() → core::int*;
+  abstract member-signature method _instanceOf(dynamic instantiatorTypeArguments, dynamic functionTypeArguments, dynamic type) → core::bool*;
+  abstract member-signature method _simpleInstanceOf(dynamic type) → core::bool*;
+  abstract member-signature method _simpleInstanceOfTrue(dynamic type) → core::bool*;
+  abstract member-signature method _simpleInstanceOfFalse(dynamic type) → core::bool*;
+  abstract member-signature operator ==(dynamic other) → core::bool*;
+  abstract member-signature get hashCode() → core::int*;
+  abstract member-signature method toString() → core::String*;
+  abstract member-signature method noSuchMethod(core::Invocation* invocation) → dynamic;
+  abstract member-signature get runtimeType() → core::Type*;
+}
+class Class3b extends self::_Class3b&C&MixinC {
+  synthetic constructor •() → self::Class3b*
+    : super self::_Class3b&C&MixinC::•()
+    ;
+}
+abstract class _Class3d&C&MixinC = bad::C<core::num*, core::int*> with bad::MixinC<core::num*, core::int*> /*isAnonymousMixin*/  {
+  synthetic constructor •() → self::_Class3d&C&MixinC*
+    : super bad::C::•()
+    ;
+  abstract member-signature get _identityHashCode() → core::int*;
+  abstract member-signature method _instanceOf(dynamic instantiatorTypeArguments, dynamic functionTypeArguments, dynamic type) → core::bool*;
+  abstract member-signature method _simpleInstanceOf(dynamic type) → core::bool*;
+  abstract member-signature method _simpleInstanceOfTrue(dynamic type) → core::bool*;
+  abstract member-signature method _simpleInstanceOfFalse(dynamic type) → core::bool*;
+  abstract member-signature operator ==(dynamic other) → core::bool*;
+  abstract member-signature get hashCode() → core::int*;
+  abstract member-signature method toString() → core::String*;
+  abstract member-signature method noSuchMethod(core::Invocation* invocation) → dynamic;
+  abstract member-signature get runtimeType() → core::Type*;
+}
+class Class3d extends self::_Class3d&C&MixinC {
+  synthetic constructor •() → self::Class3d*
+    : super self::_Class3d&C&MixinC::•()
+    ;
+}
+abstract class _Class4a&ClassBa&MixinB = bad::ClassBa with bad::MixinB<core::int*> /*isAnonymousMixin*/  {
+  synthetic constructor •() → self::_Class4a&ClassBa&MixinB*
+    : super bad::ClassBa::•()
+    ;
+  abstract member-signature get _identityHashCode() → core::int*;
+  abstract member-signature method _instanceOf(dynamic instantiatorTypeArguments, dynamic functionTypeArguments, dynamic type) → core::bool*;
+  abstract member-signature method _simpleInstanceOf(dynamic type) → core::bool*;
+  abstract member-signature method _simpleInstanceOfTrue(dynamic type) → core::bool*;
+  abstract member-signature method _simpleInstanceOfFalse(dynamic type) → core::bool*;
+  abstract member-signature operator ==(dynamic other) → core::bool*;
+  abstract member-signature get hashCode() → core::int*;
+  abstract member-signature method toString() → core::String*;
+  abstract member-signature method noSuchMethod(core::Invocation* invocation) → dynamic;
+  abstract member-signature get runtimeType() → core::Type*;
+}
+class Class4a extends self::_Class4a&ClassBa&MixinB {
+  synthetic constructor •() → self::Class4a*
+    : super self::_Class4a&ClassBa&MixinB::•()
+    ;
+}
+abstract class _Class4b&ClassBb&MixinB = bad::ClassBb with bad::MixinB<core::int*> /*isAnonymousMixin*/  {
+  synthetic constructor •() → self::_Class4b&ClassBb&MixinB*
+    : super bad::ClassBb::•()
+    ;
+  abstract member-signature get _identityHashCode() → core::int*;
+  abstract member-signature method _instanceOf(dynamic instantiatorTypeArguments, dynamic functionTypeArguments, dynamic type) → core::bool*;
+  abstract member-signature method _simpleInstanceOf(dynamic type) → core::bool*;
+  abstract member-signature method _simpleInstanceOfTrue(dynamic type) → core::bool*;
+  abstract member-signature method _simpleInstanceOfFalse(dynamic type) → core::bool*;
+  abstract member-signature operator ==(dynamic other) → core::bool*;
+  abstract member-signature get hashCode() → core::int*;
+  abstract member-signature method toString() → core::String*;
+  abstract member-signature method noSuchMethod(core::Invocation* invocation) → dynamic;
+  abstract member-signature get runtimeType() → core::Type*;
+}
+class Class4b extends self::_Class4b&ClassBb&MixinB {
+  synthetic constructor •() → self::Class4b*
+    : super self::_Class4b&ClassBb&MixinB::•()
+    ;
+}
+abstract class _Class5a&ClassCa&MixinC = bad::ClassCa with bad::MixinC<core::num*, core::int*> /*isAnonymousMixin*/  {
+  synthetic constructor •() → self::_Class5a&ClassCa&MixinC*
+    : super bad::ClassCa::•()
+    ;
+  abstract member-signature get _identityHashCode() → core::int*;
+  abstract member-signature method _instanceOf(dynamic instantiatorTypeArguments, dynamic functionTypeArguments, dynamic type) → core::bool*;
+  abstract member-signature method _simpleInstanceOf(dynamic type) → core::bool*;
+  abstract member-signature method _simpleInstanceOfTrue(dynamic type) → core::bool*;
+  abstract member-signature method _simpleInstanceOfFalse(dynamic type) → core::bool*;
+  abstract member-signature operator ==(dynamic other) → core::bool*;
+  abstract member-signature get hashCode() → core::int*;
+  abstract member-signature method toString() → core::String*;
+  abstract member-signature method noSuchMethod(core::Invocation* invocation) → dynamic;
+  abstract member-signature get runtimeType() → core::Type*;
+}
+class Class5a extends self::_Class5a&ClassCa&MixinC {
+  synthetic constructor •() → self::Class5a*
+    : super self::_Class5a&ClassCa&MixinC::•()
+    ;
+}
+abstract class _Class5b&ClassCb&MixinC = bad::ClassCb with bad::MixinC<core::num*, core::int*> /*isAnonymousMixin*/  {
+  synthetic constructor •() → self::_Class5b&ClassCb&MixinC*
+    : super bad::ClassCb::•()
+    ;
+  abstract member-signature get _identityHashCode() → core::int*;
+  abstract member-signature method _instanceOf(dynamic instantiatorTypeArguments, dynamic functionTypeArguments, dynamic type) → core::bool*;
+  abstract member-signature method _simpleInstanceOf(dynamic type) → core::bool*;
+  abstract member-signature method _simpleInstanceOfTrue(dynamic type) → core::bool*;
+  abstract member-signature method _simpleInstanceOfFalse(dynamic type) → core::bool*;
+  abstract member-signature operator ==(dynamic other) → core::bool*;
+  abstract member-signature get hashCode() → core::int*;
+  abstract member-signature method toString() → core::String*;
+  abstract member-signature method noSuchMethod(core::Invocation* invocation) → dynamic;
+  abstract member-signature get runtimeType() → core::Type*;
+}
+class Class5b extends self::_Class5b&ClassCb&MixinC {
+  synthetic constructor •() → self::Class5b*
+    : super self::_Class5b&ClassCb&MixinC::•()
+    ;
+}
+static method main() → dynamic {}
+
+library /*isNonNullableByDefault*/;
+//
+// Problems in library:
+//
+// pkg/front_end/testcases/nnbd_mixed/bad_mixins_lib.dart:17:7: Error: 'Object' doesn't implement 'A' so it can't be used with 'MixinA'.
+//  - 'Object' is from 'dart:core'.
+//  - 'A' is from 'pkg/front_end/testcases/nnbd_mixed/bad_mixins_lib.dart'.
+//  - 'MixinA' is from 'pkg/front_end/testcases/nnbd_mixed/bad_mixins_lib.dart'.
+// class Class1a extends Object with MixinA {} // error
+//       ^
+//
+// pkg/front_end/testcases/nnbd_mixed/bad_mixins_lib.dart:21:7: Error: 'Object' doesn't implement 'B<int>' so it can't be used with 'MixinB<int>'.
+//  - 'Object' is from 'dart:core'.
+//  - 'B' is from 'pkg/front_end/testcases/nnbd_mixed/bad_mixins_lib.dart'.
+//  - 'MixinB' is from 'pkg/front_end/testcases/nnbd_mixed/bad_mixins_lib.dart'.
+// class Class2a extends Object with MixinB<int> {} // error
+//       ^
+//
+// pkg/front_end/testcases/nnbd_mixed/bad_mixins_lib.dart:23:7: Error: 'B<int?>' doesn't implement 'B<int>' so it can't be used with 'MixinB<int>'.
+//  - 'B' is from 'pkg/front_end/testcases/nnbd_mixed/bad_mixins_lib.dart'.
+//  - 'MixinB' is from 'pkg/front_end/testcases/nnbd_mixed/bad_mixins_lib.dart'.
+// class Class2b extends B<int?> with MixinB<int> {} // error
+//       ^
+//
+// pkg/front_end/testcases/nnbd_mixed/bad_mixins_lib.dart:25:7: Error: 'B<num>' doesn't implement 'B<int>' so it can't be used with 'MixinB<int>'.
+//  - 'B' is from 'pkg/front_end/testcases/nnbd_mixed/bad_mixins_lib.dart'.
+//  - 'MixinB' is from 'pkg/front_end/testcases/nnbd_mixed/bad_mixins_lib.dart'.
+// class Class2c extends B<num> with MixinB<int> {} // error
+//       ^
+//
+// pkg/front_end/testcases/nnbd_mixed/bad_mixins_lib.dart:29:7: Error: 'B<Object>' doesn't implement 'B<dynamic>' so it can't be used with 'MixinB<dynamic>'.
+//  - 'B' is from 'pkg/front_end/testcases/nnbd_mixed/bad_mixins_lib.dart'.
+//  - 'Object' is from 'dart:core'.
+//  - 'MixinB' is from 'pkg/front_end/testcases/nnbd_mixed/bad_mixins_lib.dart'.
+// class Class2e extends B<Object> with MixinB<dynamic> {} // error
+//       ^
+//
+// pkg/front_end/testcases/nnbd_mixed/bad_mixins_lib.dart:33:7: Error: 'Object' doesn't implement 'C<num, int>' so it can't be used with 'MixinC<num, int>'.
+//  - 'Object' is from 'dart:core'.
+//  - 'C' is from 'pkg/front_end/testcases/nnbd_mixed/bad_mixins_lib.dart'.
+//  - 'MixinC' is from 'pkg/front_end/testcases/nnbd_mixed/bad_mixins_lib.dart'.
+// class Class3a extends Object with MixinC<num, int> {} // error
+//       ^
+//
+// pkg/front_end/testcases/nnbd_mixed/bad_mixins_lib.dart:35:7: Error: 'C<num, num>' doesn't implement 'C<num, int>' so it can't be used with 'MixinC<num, int>'.
+//  - 'C' is from 'pkg/front_end/testcases/nnbd_mixed/bad_mixins_lib.dart'.
+//  - 'MixinC' is from 'pkg/front_end/testcases/nnbd_mixed/bad_mixins_lib.dart'.
+// class Class3b extends C<num, num> with MixinC<num, int> {} // error
+//       ^
+//
+// pkg/front_end/testcases/nnbd_mixed/bad_mixins_lib.dart:37:7: Error: 'C<num?, int>' doesn't implement 'C<num, int>' so it can't be used with 'MixinC<num, int>'.
+//  - 'C' is from 'pkg/front_end/testcases/nnbd_mixed/bad_mixins_lib.dart'.
+//  - 'MixinC' is from 'pkg/front_end/testcases/nnbd_mixed/bad_mixins_lib.dart'.
+// class Class3c extends C<num?, int> with MixinC<num, int> {} // error
+//       ^
+//
+// pkg/front_end/testcases/nnbd_mixed/bad_mixins_lib.dart:49:7: Error: 'ClassBa' doesn't implement 'B<int>' so it can't be used with 'MixinB<int>'.
+//  - 'ClassBa' is from 'pkg/front_end/testcases/nnbd_mixed/bad_mixins_lib.dart'.
+//  - 'B' is from 'pkg/front_end/testcases/nnbd_mixed/bad_mixins_lib.dart'.
+//  - 'MixinB' is from 'pkg/front_end/testcases/nnbd_mixed/bad_mixins_lib.dart'.
+// class Class4a extends ClassBa with MixinB<int> {} // error
+//       ^
+//
+// pkg/front_end/testcases/nnbd_mixed/bad_mixins_lib.dart:53:7: Error: 'ClassBb' doesn't implement 'B<int?>' so it can't be used with 'MixinB<int?>'.
+//  - 'ClassBb' is from 'pkg/front_end/testcases/nnbd_mixed/bad_mixins_lib.dart'.
+//  - 'B' is from 'pkg/front_end/testcases/nnbd_mixed/bad_mixins_lib.dart'.
+//  - 'MixinB' is from 'pkg/front_end/testcases/nnbd_mixed/bad_mixins_lib.dart'.
+// class Class4c extends ClassBb with MixinB<int?> {} // error
+//       ^
+//
+// pkg/front_end/testcases/nnbd_mixed/bad_mixins_lib.dart:57:7: Error: 'ClassCa' doesn't implement 'C<num?, int>' so it can't be used with 'MixinC<num?, int>'.
+//  - 'ClassCa' is from 'pkg/front_end/testcases/nnbd_mixed/bad_mixins_lib.dart'.
+//  - 'C' is from 'pkg/front_end/testcases/nnbd_mixed/bad_mixins_lib.dart'.
+//  - 'MixinC' is from 'pkg/front_end/testcases/nnbd_mixed/bad_mixins_lib.dart'.
+// class Class5a extends ClassCa with MixinC<num?, int> {} // error
+//       ^
+//
+// pkg/front_end/testcases/nnbd_mixed/bad_mixins_lib.dart:61:7: Error: 'ClassCb' doesn't implement 'C<num?, int?>' so it can't be used with 'MixinC<num?, int?>'.
+//  - 'ClassCb' is from 'pkg/front_end/testcases/nnbd_mixed/bad_mixins_lib.dart'.
+//  - 'C' is from 'pkg/front_end/testcases/nnbd_mixed/bad_mixins_lib.dart'.
+//  - 'MixinC' is from 'pkg/front_end/testcases/nnbd_mixed/bad_mixins_lib.dart'.
+// class Class5c extends ClassCb with MixinC<num?, int?> {} // error
+//       ^
+//
+import self as bad;
+import "dart:core" as core;
+
+class A extends core::Object {
+  synthetic constructor •() → bad::A
+    : super core::Object::•()
+    ;
+}
+class B<T extends core::Object? = dynamic> extends core::Object {
+  synthetic constructor •() → bad::B<bad::B::T%>
+    : super core::Object::•()
+    ;
+}
+class C<T extends core::Object? = dynamic, S extends bad::C::T% = dynamic> extends core::Object {
+  synthetic constructor •() → bad::C<bad::C::T%, bad::C::S%>
+    : super core::Object::•()
+    ;
+}
+abstract class MixinA extends bad::A /*isMixinDeclaration*/  {
+}
+abstract class MixinB<T extends core::Object? = dynamic> extends bad::B<bad::MixinB::T%> /*isMixinDeclaration*/  {
+}
+abstract class MixinC<T extends core::Object? = dynamic, S extends bad::MixinC::T% = dynamic> extends bad::C<bad::MixinC::T%, bad::MixinC::S%> /*isMixinDeclaration*/  {
+}
+abstract class _Class1a&Object&MixinA = core::Object with bad::MixinA /*isAnonymousMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → bad::_Class1a&Object&MixinA
+    : super core::Object::•()
+    ;
+}
+class Class1a extends bad::_Class1a&Object&MixinA {
+  synthetic constructor •() → bad::Class1a
+    : super bad::_Class1a&Object&MixinA::•()
+    ;
+}
+abstract class _Class1b&A&MixinA = bad::A with bad::MixinA /*isAnonymousMixin*/  {
+  synthetic constructor •() → bad::_Class1b&A&MixinA
+    : super bad::A::•()
+    ;
+}
+class Class1b extends bad::_Class1b&A&MixinA {
+  synthetic constructor •() → bad::Class1b
+    : super bad::_Class1b&A&MixinA::•()
+    ;
+}
+abstract class _Class2a&Object&MixinB = core::Object with bad::MixinB<core::int> /*isAnonymousMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → bad::_Class2a&Object&MixinB
+    : super core::Object::•()
+    ;
+}
+class Class2a extends bad::_Class2a&Object&MixinB {
+  synthetic constructor •() → bad::Class2a
+    : super bad::_Class2a&Object&MixinB::•()
+    ;
+}
+abstract class _Class2b&B&MixinB = bad::B<core::int?> with bad::MixinB<core::int> /*isAnonymousMixin*/  {
+  synthetic constructor •() → bad::_Class2b&B&MixinB
+    : super bad::B::•()
+    ;
+}
+class Class2b extends bad::_Class2b&B&MixinB {
+  synthetic constructor •() → bad::Class2b
+    : super bad::_Class2b&B&MixinB::•()
+    ;
+}
+abstract class _Class2c&B&MixinB = bad::B<core::num> with bad::MixinB<core::int> /*isAnonymousMixin*/  {
+  synthetic constructor •() → bad::_Class2c&B&MixinB
+    : super bad::B::•()
+    ;
+}
+class Class2c extends bad::_Class2c&B&MixinB {
+  synthetic constructor •() → bad::Class2c
+    : super bad::_Class2c&B&MixinB::•()
+    ;
+}
+abstract class _Class2d&B&MixinB = bad::B<core::int> with bad::MixinB<core::int> /*isAnonymousMixin*/  {
+  synthetic constructor •() → bad::_Class2d&B&MixinB
+    : super bad::B::•()
+    ;
+}
+class Class2d extends bad::_Class2d&B&MixinB {
+  synthetic constructor •() → bad::Class2d
+    : super bad::_Class2d&B&MixinB::•()
+    ;
+}
+abstract class _Class2e&B&MixinB = bad::B<core::Object> with bad::MixinB<dynamic> /*isAnonymousMixin*/  {
+  synthetic constructor •() → bad::_Class2e&B&MixinB
+    : super bad::B::•()
+    ;
+}
+class Class2e extends bad::_Class2e&B&MixinB {
+  synthetic constructor •() → bad::Class2e
+    : super bad::_Class2e&B&MixinB::•()
+    ;
+}
+abstract class _Class2f&B&MixinB = bad::B<core::Object?> with bad::MixinB<dynamic> /*isAnonymousMixin*/  {
+  synthetic constructor •() → bad::_Class2f&B&MixinB
+    : super bad::B::•()
+    ;
+}
+class Class2f extends bad::_Class2f&B&MixinB {
+  synthetic constructor •() → bad::Class2f
+    : super bad::_Class2f&B&MixinB::•()
+    ;
+}
+abstract class _Class3a&Object&MixinC = core::Object with bad::MixinC<core::num, core::int> /*isAnonymousMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → bad::_Class3a&Object&MixinC
+    : super core::Object::•()
+    ;
+}
+class Class3a extends bad::_Class3a&Object&MixinC {
+  synthetic constructor •() → bad::Class3a
+    : super bad::_Class3a&Object&MixinC::•()
+    ;
+}
+abstract class _Class3b&C&MixinC = bad::C<core::num, core::num> with bad::MixinC<core::num, core::int> /*isAnonymousMixin*/  {
+  synthetic constructor •() → bad::_Class3b&C&MixinC
+    : super bad::C::•()
+    ;
+}
+class Class3b extends bad::_Class3b&C&MixinC {
+  synthetic constructor •() → bad::Class3b
+    : super bad::_Class3b&C&MixinC::•()
+    ;
+}
+abstract class _Class3c&C&MixinC = bad::C<core::num?, core::int> with bad::MixinC<core::num, core::int> /*isAnonymousMixin*/  {
+  synthetic constructor •() → bad::_Class3c&C&MixinC
+    : super bad::C::•()
+    ;
+}
+class Class3c extends bad::_Class3c&C&MixinC {
+  synthetic constructor •() → bad::Class3c
+    : super bad::_Class3c&C&MixinC::•()
+    ;
+}
+abstract class _Class3d&C&MixinC = bad::C<core::num, core::int> with bad::MixinC<core::num, core::int> /*isAnonymousMixin*/  {
+  synthetic constructor •() → bad::_Class3d&C&MixinC
+    : super bad::C::•()
+    ;
+}
+class Class3d extends bad::_Class3d&C&MixinC {
+  synthetic constructor •() → bad::Class3d
+    : super bad::_Class3d&C&MixinC::•()
+    ;
+}
+class ClassBa extends bad::B<core::int?> {
+  synthetic constructor •() → bad::ClassBa
+    : super bad::B::•()
+    ;
+}
+class ClassBb extends bad::B<core::int> {
+  synthetic constructor •() → bad::ClassBb
+    : super bad::B::•()
+    ;
+}
+class ClassCa extends bad::C<core::num?, core::int?> {
+  synthetic constructor •() → bad::ClassCa
+    : super bad::C::•()
+    ;
+}
+class ClassCb extends bad::C<core::num?, core::int> {
+  synthetic constructor •() → bad::ClassCb
+    : super bad::C::•()
+    ;
+}
+abstract class _Class4a&ClassBa&MixinB = bad::ClassBa with bad::MixinB<core::int> /*isAnonymousMixin*/  {
+  synthetic constructor •() → bad::_Class4a&ClassBa&MixinB
+    : super bad::ClassBa::•()
+    ;
+}
+class Class4a extends bad::_Class4a&ClassBa&MixinB {
+  synthetic constructor •() → bad::Class4a
+    : super bad::_Class4a&ClassBa&MixinB::•()
+    ;
+}
+abstract class _Class4b&ClassBa&MixinB = bad::ClassBa with bad::MixinB<core::int?> /*isAnonymousMixin*/  {
+  synthetic constructor •() → bad::_Class4b&ClassBa&MixinB
+    : super bad::ClassBa::•()
+    ;
+}
+class Class4b extends bad::_Class4b&ClassBa&MixinB {
+  synthetic constructor •() → bad::Class4b
+    : super bad::_Class4b&ClassBa&MixinB::•()
+    ;
+}
+abstract class _Class4c&ClassBb&MixinB = bad::ClassBb with bad::MixinB<core::int?> /*isAnonymousMixin*/  {
+  synthetic constructor •() → bad::_Class4c&ClassBb&MixinB
+    : super bad::ClassBb::•()
+    ;
+}
+class Class4c extends bad::_Class4c&ClassBb&MixinB {
+  synthetic constructor •() → bad::Class4c
+    : super bad::_Class4c&ClassBb&MixinB::•()
+    ;
+}
+abstract class _Class4d&ClassBb&MixinB = bad::ClassBb with bad::MixinB<core::int> /*isAnonymousMixin*/  {
+  synthetic constructor •() → bad::_Class4d&ClassBb&MixinB
+    : super bad::ClassBb::•()
+    ;
+}
+class Class4d extends bad::_Class4d&ClassBb&MixinB {
+  synthetic constructor •() → bad::Class4d
+    : super bad::_Class4d&ClassBb&MixinB::•()
+    ;
+}
+abstract class _Class5a&ClassCa&MixinC = bad::ClassCa with bad::MixinC<core::num?, core::int> /*isAnonymousMixin*/  {
+  synthetic constructor •() → bad::_Class5a&ClassCa&MixinC
+    : super bad::ClassCa::•()
+    ;
+}
+class Class5a extends bad::_Class5a&ClassCa&MixinC {
+  synthetic constructor •() → bad::Class5a
+    : super bad::_Class5a&ClassCa&MixinC::•()
+    ;
+}
+abstract class _Class5b&ClassCa&MixinC = bad::ClassCa with bad::MixinC<core::num?, core::int?> /*isAnonymousMixin*/  {
+  synthetic constructor •() → bad::_Class5b&ClassCa&MixinC
+    : super bad::ClassCa::•()
+    ;
+}
+class Class5b extends bad::_Class5b&ClassCa&MixinC {
+  synthetic constructor •() → bad::Class5b
+    : super bad::_Class5b&ClassCa&MixinC::•()
+    ;
+}
+abstract class _Class5c&ClassCb&MixinC = bad::ClassCb with bad::MixinC<core::num?, core::int?> /*isAnonymousMixin*/  {
+  synthetic constructor •() → bad::_Class5c&ClassCb&MixinC
+    : super bad::ClassCb::•()
+    ;
+}
+class Class5c extends bad::_Class5c&ClassCb&MixinC {
+  synthetic constructor •() → bad::Class5c
+    : super bad::_Class5c&ClassCb&MixinC::•()
+    ;
+}
+abstract class _Class5d&ClassCb&MixinC = bad::ClassCb with bad::MixinC<core::num?, core::int> /*isAnonymousMixin*/  {
+  synthetic constructor •() → bad::_Class5d&ClassCb&MixinC
+    : super bad::ClassCb::•()
+    ;
+}
+class Class5d extends bad::_Class5d&ClassCb&MixinC {
+  synthetic constructor •() → bad::Class5d
+    : super bad::_Class5d&ClassCb&MixinC::•()
+    ;
+}
diff --git a/pkg/front_end/testcases/nnbd_mixed/bad_mixins_lib.dart b/pkg/front_end/testcases/nnbd_mixed/bad_mixins_lib.dart
new file mode 100644
index 0000000..3e0b43b
--- /dev/null
+++ b/pkg/front_end/testcases/nnbd_mixed/bad_mixins_lib.dart
@@ -0,0 +1,63 @@
+// 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.
+
+class A {}
+
+class B<T> {}
+
+class C<T, S extends T> {}
+
+mixin MixinA on A {}
+
+mixin MixinB<T> on B<T> {}
+
+mixin MixinC<T, S extends T> on C<T, S> {}
+
+class Class1a extends Object with MixinA {} // error
+
+class Class1b extends A with MixinA {} // ok
+
+class Class2a extends Object with MixinB<int> {} // error
+
+class Class2b extends B<int?> with MixinB<int> {} // error
+
+class Class2c extends B<num> with MixinB<int> {} // error
+
+class Class2d extends B<int> with MixinB<int> {} // ok
+
+class Class2e extends B<Object> with MixinB<dynamic> {} // error
+
+class Class2f extends B<Object?> with MixinB<dynamic> {} // ok
+
+class Class3a extends Object with MixinC<num, int> {} // error
+
+class Class3b extends C<num, num> with MixinC<num, int> {} // error
+
+class Class3c extends C<num?, int> with MixinC<num, int> {} // error
+
+class Class3d extends C<num, int> with MixinC<num, int> {} // ok
+
+class ClassBa extends B<int?> {}
+
+class ClassBb extends B<int> {}
+
+class ClassCa extends C<num?, int?> {}
+
+class ClassCb extends C<num?, int> {}
+
+class Class4a extends ClassBa with MixinB<int> {} // error
+
+class Class4b extends ClassBa with MixinB<int?> {} // ok
+
+class Class4c extends ClassBb with MixinB<int?> {} // error
+
+class Class4d extends ClassBb with MixinB<int> {} // ok
+
+class Class5a extends ClassCa with MixinC<num?, int> {} // error
+
+class Class5b extends ClassCa with MixinC<num?, int?> {} // ok
+
+class Class5c extends ClassCb with MixinC<num?, int?> {} // error
+
+class Class5d extends ClassCb with MixinC<num?, int> {} // ok
diff --git a/pkg/front_end/testcases/nnbd_mixed/flutter_issue_63029/flutter_issue_63029.dart b/pkg/front_end/testcases/nnbd_mixed/flutter_issue_63029/flutter_issue_63029.dart
index 3e5c10c..2d9f496 100644
--- a/pkg/front_end/testcases/nnbd_mixed/flutter_issue_63029/flutter_issue_63029.dart
+++ b/pkg/front_end/testcases/nnbd_mixed/flutter_issue_63029/flutter_issue_63029.dart
@@ -1,3 +1,7 @@
+// 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 'flutter_issue_63029_lib1.dart';
 import 'flutter_issue_63029_lib2.dart';
 
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 e8fe043..01c3d59 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
@@ -1,14 +1,4 @@
 library /*isNonNullableByDefault*/;
-//
-// Problems in library:
-//
-// pkg/front_end/testcases/nnbd_mixed/flutter_issue_63029/flutter_issue_63029.dart:6:7: Error: 'B<E>' doesn't implement 'B<E>' so it can't be used with 'D<E>'.
-//  - 'B' is from 'pkg/front_end/testcases/nnbd_mixed/flutter_issue_63029/flutter_issue_63029_lib1.dart'.
-//  - 'E' is from 'pkg/front_end/testcases/nnbd_mixed/flutter_issue_63029/flutter_issue_63029.dart'.
-//  - 'D' is from 'pkg/front_end/testcases/nnbd_mixed/flutter_issue_63029/flutter_issue_63029_lib2.dart'.
-// class F extends B<E> with D<E> {}
-//       ^
-//
 import self as self;
 import "flutter_issue_63029_lib1.dart" as flu;
 import "flutter_issue_63029_lib2.dart" as flu2;
diff --git a/pkg/front_end/testcases/nnbd_mixed/flutter_issue_63029/flutter_issue_63029.dart.weak.transformed.expect b/pkg/front_end/testcases/nnbd_mixed/flutter_issue_63029/flutter_issue_63029.dart.weak.transformed.expect
index f39dd6f..2e167fd 100644
--- a/pkg/front_end/testcases/nnbd_mixed/flutter_issue_63029/flutter_issue_63029.dart.weak.transformed.expect
+++ b/pkg/front_end/testcases/nnbd_mixed/flutter_issue_63029/flutter_issue_63029.dart.weak.transformed.expect
@@ -1,14 +1,4 @@
 library /*isNonNullableByDefault*/;
-//
-// Problems in library:
-//
-// pkg/front_end/testcases/nnbd_mixed/flutter_issue_63029/flutter_issue_63029.dart:6:7: Error: 'B<E>' doesn't implement 'B<E>' so it can't be used with 'D<E>'.
-//  - 'B' is from 'pkg/front_end/testcases/nnbd_mixed/flutter_issue_63029/flutter_issue_63029_lib1.dart'.
-//  - 'E' is from 'pkg/front_end/testcases/nnbd_mixed/flutter_issue_63029/flutter_issue_63029.dart'.
-//  - 'D' is from 'pkg/front_end/testcases/nnbd_mixed/flutter_issue_63029/flutter_issue_63029_lib2.dart'.
-// class F extends B<E> with D<E> {}
-//       ^
-//
 import self as self;
 import "flutter_issue_63029_lib1.dart" as flu;
 import "flutter_issue_63029_lib2.dart" as flu2;
diff --git a/pkg/front_end/testcases/nnbd_mixed/flutter_issue_63029/flutter_issue_63029_lib1.dart b/pkg/front_end/testcases/nnbd_mixed/flutter_issue_63029/flutter_issue_63029_lib1.dart
index 90bdb8f..e709a65 100644
--- a/pkg/front_end/testcases/nnbd_mixed/flutter_issue_63029/flutter_issue_63029_lib1.dart
+++ b/pkg/front_end/testcases/nnbd_mixed/flutter_issue_63029/flutter_issue_63029_lib1.dart
@@ -1,3 +1,7 @@
+// 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.
+
 abstract class A {}
 
 abstract class B<T extends A> {}
diff --git a/pkg/front_end/testcases/nnbd_mixed/flutter_issue_63029/flutter_issue_63029_lib2.dart b/pkg/front_end/testcases/nnbd_mixed/flutter_issue_63029/flutter_issue_63029_lib2.dart
index 7205587..c273013 100644
--- a/pkg/front_end/testcases/nnbd_mixed/flutter_issue_63029/flutter_issue_63029_lib2.dart
+++ b/pkg/front_end/testcases/nnbd_mixed/flutter_issue_63029/flutter_issue_63029_lib2.dart
@@ -1,3 +1,7 @@
+// 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 = 2.8
 
 import 'flutter_issue_63029_lib1.dart';
diff --git a/pkg/front_end/testcases/weak.status b/pkg/front_end/testcases/weak.status
index 65846ca..93d5a75 100644
--- a/pkg/front_end/testcases/weak.status
+++ b/pkg/front_end/testcases/weak.status
@@ -65,6 +65,7 @@
 nnbd/nullable_object_access: TypeCheckError
 nnbd/nullable_receiver: TypeCheckError
 nnbd/potentially_nullable_access: TypeCheckError
+nnbd_mixed/bad_mixins: TypeCheckError
 nnbd_mixed/inheritance_from_opt_in: TypeCheckError
 nnbd_mixed/issue41567: TypeCheckError
 nnbd_mixed/messages_with_types_opt_in: TypeCheckError
diff --git a/tools/VERSION b/tools/VERSION
index e885501..e7d799b 100644
--- a/tools/VERSION
+++ b/tools/VERSION
@@ -27,5 +27,5 @@
 MAJOR 2
 MINOR 10
 PATCH 0
-PRERELEASE 10
+PRERELEASE 11
 PRERELEASE_PATCH 0
\ No newline at end of file