diff --git a/pkg/front_end/lib/src/fasta/kernel/class_hierarchy_builder.dart b/pkg/front_end/lib/src/fasta/kernel/class_hierarchy_builder.dart
index 0046492..29e6bd3 100644
--- a/pkg/front_end/lib/src/fasta/kernel/class_hierarchy_builder.dart
+++ b/pkg/front_end/lib/src/fasta/kernel/class_hierarchy_builder.dart
@@ -523,6 +523,10 @@
     for (int i = 0; i < nodes2.length; i++) {
       ClassHierarchyNode node = nodes2[i];
       if (node == null) continue;
+      if (node.classBuilder.cls.isAnonymousMixin) {
+        // Never find unnamed mixin application in least upper bound.
+        continue;
+      }
       if (nodes1.contains(node)) {
         DartType candidate1 = getTypeAsInstanceOf(
             type1, node.classBuilder.cls, clientLibrary, coreTypes);
diff --git a/pkg/front_end/test/spell_checking_list_tests.txt b/pkg/front_end/test/spell_checking_list_tests.txt
index a825e87..f858baf 100644
--- a/pkg/front_end/test/spell_checking_list_tests.txt
+++ b/pkg/front_end/test/spell_checking_list_tests.txt
@@ -151,6 +151,7 @@
 decrease
 decrements
 dectcem
+deduplication
 defaulting
 defintions
 deleting
@@ -163,6 +164,7 @@
 deviation
 dfast
 di
+diagnosticable
 dictionaries
 dictionary
 differences
@@ -216,6 +218,7 @@
 execute
 exercised
 exercises
+existence
 exitcode
 exiting
 expanded
diff --git a/pkg/front_end/testcases/general/mixin_application_lub.dart b/pkg/front_end/testcases/general/mixin_application_lub.dart
new file mode 100644
index 0000000..a144b92
--- /dev/null
+++ b/pkg/front_end/testcases/general/mixin_application_lub.dart
@@ -0,0 +1,31 @@
+// 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 Diagnosticable {}
+
+// Originally the existence of this (unused) class (which has an anonymous mixin
+// that matches that of `State<T>`) caused an error in the VM's mixin
+// deduplication code. This was due to the inferred type for `var x = a ?? b`
+// was the anonymous mixin application that got removed during deduplication.
+//
+// See https://github.com/flutter/flutter/issues/55345
+class SomeClass with Diagnosticable {}
+
+class State<T> with Diagnosticable {}
+
+class StateA extends State {}
+
+class StateB extends State<int> {}
+
+StateA a = StateA();
+StateB b = StateB();
+
+foo<T>(T x) {
+  print(T);
+}
+
+main() {
+  var x = a ?? b;
+  foo(x);
+}
diff --git a/pkg/front_end/testcases/general/mixin_application_lub.dart.outline.expect b/pkg/front_end/testcases/general/mixin_application_lub.dart.outline.expect
new file mode 100644
index 0000000..cf58674
--- /dev/null
+++ b/pkg/front_end/testcases/general/mixin_application_lub.dart.outline.expect
@@ -0,0 +1,70 @@
+library;
+import self as self;
+import "dart:core" as core;
+
+class Diagnosticable extends core::Object {
+  synthetic constructor •() → self::Diagnosticable*
+    ;
+  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
+}
+abstract class _SomeClass&Object&Diagnosticable = core::Object with self::Diagnosticable /*isAnonymousMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_SomeClass&Object&Diagnosticable*
+    : super core::Object::•()
+    ;
+  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
+}
+class SomeClass extends self::_SomeClass&Object&Diagnosticable {
+  synthetic constructor •() → self::SomeClass*
+    ;
+}
+abstract class _State&Object&Diagnosticable = core::Object with self::Diagnosticable /*isAnonymousMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_State&Object&Diagnosticable*
+    : super core::Object::•()
+    ;
+  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
+}
+class State<T extends core::Object* = dynamic> extends self::_State&Object&Diagnosticable {
+  synthetic constructor •() → self::State<self::State::T*>*
+    ;
+}
+class StateA extends self::State<dynamic> {
+  synthetic constructor •() → self::StateA*
+    ;
+}
+class StateB extends self::State<core::int*> {
+  synthetic constructor •() → self::StateB*
+    ;
+}
+static field self::StateA* a;
+static field self::StateB* b;
+static method foo<T extends core::Object* = dynamic>(self::foo::T* x) → dynamic
+  ;
+static method main() → dynamic
+  ;
diff --git a/pkg/front_end/testcases/general/mixin_application_lub.dart.strong.expect b/pkg/front_end/testcases/general/mixin_application_lub.dart.strong.expect
new file mode 100644
index 0000000..e277693
--- /dev/null
+++ b/pkg/front_end/testcases/general/mixin_application_lub.dart.strong.expect
@@ -0,0 +1,78 @@
+library;
+import self as self;
+import "dart:core" as core;
+
+class Diagnosticable extends core::Object {
+  synthetic constructor •() → self::Diagnosticable*
+    : super core::Object::•()
+    ;
+  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
+}
+abstract class _SomeClass&Object&Diagnosticable = core::Object with self::Diagnosticable /*isAnonymousMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_SomeClass&Object&Diagnosticable*
+    : super core::Object::•()
+    ;
+  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
+}
+class SomeClass extends self::_SomeClass&Object&Diagnosticable {
+  synthetic constructor •() → self::SomeClass*
+    : super self::_SomeClass&Object&Diagnosticable::•()
+    ;
+}
+abstract class _State&Object&Diagnosticable = core::Object with self::Diagnosticable /*isAnonymousMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_State&Object&Diagnosticable*
+    : super core::Object::•()
+    ;
+  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
+}
+class State<T extends core::Object* = dynamic> extends self::_State&Object&Diagnosticable {
+  synthetic constructor •() → self::State<self::State::T*>*
+    : super self::_State&Object&Diagnosticable::•()
+    ;
+}
+class StateA extends self::State<dynamic> {
+  synthetic constructor •() → self::StateA*
+    : super self::State::•()
+    ;
+}
+class StateB extends self::State<core::int*> {
+  synthetic constructor •() → self::StateB*
+    : super self::State::•()
+    ;
+}
+static field self::StateA* a = new self::StateA::•();
+static field self::StateB* b = new self::StateB::•();
+static method foo<T extends core::Object* = dynamic>(self::foo::T* x) → dynamic {
+  core::print(self::foo::T*);
+}
+static method main() → dynamic {
+  self::Diagnosticable* x = let final self::StateA* #t1 = self::a in #t1.{self::_State&Object&Diagnosticable::==}(null) ?{self::Diagnosticable*} self::b : #t1;
+  self::foo<self::Diagnosticable*>(x);
+}
diff --git a/pkg/front_end/testcases/general/mixin_application_lub.dart.strong.transformed.expect b/pkg/front_end/testcases/general/mixin_application_lub.dart.strong.transformed.expect
new file mode 100644
index 0000000..421798b
--- /dev/null
+++ b/pkg/front_end/testcases/general/mixin_application_lub.dart.strong.transformed.expect
@@ -0,0 +1,78 @@
+library;
+import self as self;
+import "dart:core" as core;
+
+class Diagnosticable extends core::Object {
+  synthetic constructor •() → self::Diagnosticable*
+    : super core::Object::•()
+    ;
+  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
+}
+abstract class _SomeClass&Object&Diagnosticable extends core::Object implements self::Diagnosticable /*isAnonymousMixin,isEliminatedMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_SomeClass&Object&Diagnosticable*
+    : super core::Object::•()
+    ;
+  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
+}
+class SomeClass extends self::_SomeClass&Object&Diagnosticable {
+  synthetic constructor •() → self::SomeClass*
+    : super self::_SomeClass&Object&Diagnosticable::•()
+    ;
+}
+abstract class _State&Object&Diagnosticable extends core::Object implements self::Diagnosticable /*isAnonymousMixin,isEliminatedMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_State&Object&Diagnosticable*
+    : super core::Object::•()
+    ;
+  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
+}
+class State<T extends core::Object* = dynamic> extends self::_State&Object&Diagnosticable {
+  synthetic constructor •() → self::State<self::State::T*>*
+    : super self::_State&Object&Diagnosticable::•()
+    ;
+}
+class StateA extends self::State<dynamic> {
+  synthetic constructor •() → self::StateA*
+    : super self::State::•()
+    ;
+}
+class StateB extends self::State<core::int*> {
+  synthetic constructor •() → self::StateB*
+    : super self::State::•()
+    ;
+}
+static field self::StateA* a = new self::StateA::•();
+static field self::StateB* b = new self::StateB::•();
+static method foo<T extends core::Object* = dynamic>(self::foo::T* x) → dynamic {
+  core::print(self::foo::T*);
+}
+static method main() → dynamic {
+  self::Diagnosticable* x = let final self::StateA* #t1 = self::a in #t1.{self::_State&Object&Diagnosticable::==}(null) ?{self::Diagnosticable*} self::b : #t1;
+  self::foo<self::Diagnosticable*>(x);
+}
diff --git a/pkg/front_end/testcases/general/mixin_application_lub.dart.textual_outline.expect b/pkg/front_end/testcases/general/mixin_application_lub.dart.textual_outline.expect
new file mode 100644
index 0000000..5603659
--- /dev/null
+++ b/pkg/front_end/testcases/general/mixin_application_lub.dart.textual_outline.expect
@@ -0,0 +1,14 @@
+class Diagnosticable {}
+
+class SomeClass with Diagnosticable {}
+
+class State<T> with Diagnosticable {}
+
+class StateA extends State {}
+
+class StateB extends State<int> {}
+
+StateA a = StateA();
+StateB b = StateB();
+foo<T>(T x) {}
+main() {}
diff --git a/pkg/front_end/testcases/general/mixin_application_lub.dart.textual_outline_modelled.expect b/pkg/front_end/testcases/general/mixin_application_lub.dart.textual_outline_modelled.expect
new file mode 100644
index 0000000..0d56a51
--- /dev/null
+++ b/pkg/front_end/testcases/general/mixin_application_lub.dart.textual_outline_modelled.expect
@@ -0,0 +1,15 @@
+StateA a = StateA();
+StateB b = StateB();
+
+class Diagnosticable {}
+
+class SomeClass with Diagnosticable {}
+
+class State<T> with Diagnosticable {}
+
+class StateA extends State {}
+
+class StateB extends State<int> {}
+
+foo<T>(T x) {}
+main() {}
diff --git a/pkg/kernel/lib/class_hierarchy.dart b/pkg/kernel/lib/class_hierarchy.dart
index e461ce3..446a07f 100644
--- a/pkg/kernel/lib/class_hierarchy.dart
+++ b/pkg/kernel/lib/class_hierarchy.dart
@@ -635,8 +635,14 @@
       }
       ++i2;
       ++i1;
+      if (next.classNode.isAnonymousMixin) {
+        // Never find unnamed mixin application in least upper bound.
+        continue;
+      }
       if (next.depth != currentDepth) {
-        if (numCandidatesAtThisDepth == 1) return candidate;
+        if (numCandidatesAtThisDepth == 1) {
+          return candidate;
+        }
         currentDepth = next.depth;
         numCandidatesAtThisDepth = 0;
         candidate = null;
diff --git a/pkg/kernel/lib/verifier.dart b/pkg/kernel/lib/verifier.dart
index a826d66..5ad6c85 100644
--- a/pkg/kernel/lib/verifier.dart
+++ b/pkg/kernel/lib/verifier.dart
@@ -796,6 +796,21 @@
           " type arguments, but the class declares"
           " ${node.classNode.typeParameters.length} parameters.");
     }
+    if (node.classNode.isAnonymousMixin) {
+      if (currentParent is FunctionNode) {
+        TreeNode functionNodeParent = currentParent.parent;
+        if (functionNodeParent is Constructor ||
+            functionNodeParent is Procedure &&
+                functionNodeParent.kind == ProcedureKind.Factory) {
+          if (functionNodeParent.parent == node.classNode) {
+            // We only allow references to anonymous mixins in types as the
+            // return type of its own constructor.
+            return;
+          }
+        }
+      }
+      problem(currentParent, "Type $node references an anonymous mixin class.");
+    }
   }
 
   @override