diff --git a/tests/language/mixin_constructor_forwarding/mixin_constructor_parameter_forwarding_helper.dart b/tests/language/mixin_constructor_forwarding/mixin_constructor_parameter_forwarding_helper.dart
new file mode 100644
index 00000000000..58d2081a3e0
--- /dev/null
+++ b/tests/language/mixin_constructor_forwarding/mixin_constructor_parameter_forwarding_helper.dart
@@ -0,0 +1,23 @@
+// Copyright (c) 2019, 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 "mixin_constructor_parameter_forwarding_test.dart";
+
+// A private class that the mixin application cannot access syntactically,
+// yet it needs an instance of it for the default value.
+class _Private {
+  const _Private();
+}
+
+class B2<T> implements B<T> {
+  final T x;
+  final Object y;
+  const B2(T x, [Object y = const _Private()])
+      : x = x,
+        y = y;
+}
+
+// Leaking the constant value in a non-constant way which cannot be used
+// for the default value of the mixin application.
+Object get privateValue => const _Private();
diff --git a/tests/language/mixin_constructor_forwarding/mixin_constructor_parameter_forwarding_test.dart b/tests/language/mixin_constructor_forwarding/mixin_constructor_parameter_forwarding_test.dart
new file mode 100644
index 00000000000..0aba8b9ec79
--- /dev/null
+++ b/tests/language/mixin_constructor_forwarding/mixin_constructor_parameter_forwarding_test.dart
@@ -0,0 +1,77 @@
+// Copyright (c) 2019, 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.
+
+// Tests that mixin application forwarding constructors correctly forward
+// optional parameter default values.
+
+import "package:expect/expect.dart";
+
+import "mixin_constructor_parameter_forwarding_helper.dart"
+    show B2, privateValue;
+
+class B<T> {
+  final T x;
+  final Object y;
+  const B(T x, [Object y = 0])
+      : x = x,
+        y = y;
+  const B.b1(T x, {Object y = 0})
+      : x = x,
+        y = y;
+  const B.b2(T x, [Object y = const <int>[0]])
+      : x = x,
+        y = y;
+}
+
+mixin M1 on B<int> {
+  void check(int x, Object y) {
+    Expect.identical(x, this.x);
+    Expect.identical(y, this.y);
+  }
+}
+
+mixin M2<T> on B<T> {
+  void check(T x, Object y) {
+    Expect.identical(x, this.x);
+    Expect.identical(y, this.y);
+  }
+}
+
+class A1 = B<int> with M1;
+class A2 = B<int> with M2<int>;
+class P1 = B2<int> with M2<int>;
+
+main() {
+  A1(1, 2).check(1, 2);
+  A1.b1(1, y: 2).check(1, 2);
+  A1.b2(1, 2).check(1, 2);
+  A2(1, 2).check(1, 2);
+  A2.b1(1, y: 2).check(1, 2);
+  A2.b2(1, 2).check(1, 2);
+  P1(1, 2).check(1, 2);
+
+  A1(1).check(1, 0);
+  A1.b1(1).check(1, 0);
+  A1.b2(1).check(1, const <int>[0]);
+  A2(1).check(1, 0);
+  A2.b1(1).check(1, 0);
+  A2.b2(1).check(1, const <int>[0]);
+  P1(1).check(1, privateValue);
+
+  const A1(1, 2).check(1, 2);
+  const A1.b1(1, y: 2).check(1, 2);
+  const A1.b2(1, 2).check(1, 2);
+  const A2(1, 2).check(1, 2);
+  const A2.b1(1, y: 2).check(1, 2);
+  const A2.b2(1, 2).check(1, 2);
+  const P1(1, 2).check(1, 2);
+
+  const A1(1).check(1, 0);
+  const A1.b1(1).check(1, 0);
+  const A1.b2(1).check(1, const <int>[0]);
+  const A2(1).check(1, 0);
+  const A2.b1(1).check(1, 0);
+  const A2.b2(1).check(1, const <int>[0]);
+  const P1(1).check(1, privateValue);
+}
diff --git a/tests/language_2/mixin_constructor_forwarding/mixin_constructor_parameter_forwarding_helper.dart b/tests/language_2/mixin_constructor_forwarding/mixin_constructor_parameter_forwarding_helper.dart
new file mode 100644
index 00000000000..58d2081a3e0
--- /dev/null
+++ b/tests/language_2/mixin_constructor_forwarding/mixin_constructor_parameter_forwarding_helper.dart
@@ -0,0 +1,23 @@
+// Copyright (c) 2019, 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 "mixin_constructor_parameter_forwarding_test.dart";
+
+// A private class that the mixin application cannot access syntactically,
+// yet it needs an instance of it for the default value.
+class _Private {
+  const _Private();
+}
+
+class B2<T> implements B<T> {
+  final T x;
+  final Object y;
+  const B2(T x, [Object y = const _Private()])
+      : x = x,
+        y = y;
+}
+
+// Leaking the constant value in a non-constant way which cannot be used
+// for the default value of the mixin application.
+Object get privateValue => const _Private();
diff --git a/tests/language_2/mixin_constructor_forwarding/mixin_constructor_parameter_forwarding_test.dart b/tests/language_2/mixin_constructor_forwarding/mixin_constructor_parameter_forwarding_test.dart
new file mode 100644
index 00000000000..0aba8b9ec79
--- /dev/null
+++ b/tests/language_2/mixin_constructor_forwarding/mixin_constructor_parameter_forwarding_test.dart
@@ -0,0 +1,77 @@
+// Copyright (c) 2019, 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.
+
+// Tests that mixin application forwarding constructors correctly forward
+// optional parameter default values.
+
+import "package:expect/expect.dart";
+
+import "mixin_constructor_parameter_forwarding_helper.dart"
+    show B2, privateValue;
+
+class B<T> {
+  final T x;
+  final Object y;
+  const B(T x, [Object y = 0])
+      : x = x,
+        y = y;
+  const B.b1(T x, {Object y = 0})
+      : x = x,
+        y = y;
+  const B.b2(T x, [Object y = const <int>[0]])
+      : x = x,
+        y = y;
+}
+
+mixin M1 on B<int> {
+  void check(int x, Object y) {
+    Expect.identical(x, this.x);
+    Expect.identical(y, this.y);
+  }
+}
+
+mixin M2<T> on B<T> {
+  void check(T x, Object y) {
+    Expect.identical(x, this.x);
+    Expect.identical(y, this.y);
+  }
+}
+
+class A1 = B<int> with M1;
+class A2 = B<int> with M2<int>;
+class P1 = B2<int> with M2<int>;
+
+main() {
+  A1(1, 2).check(1, 2);
+  A1.b1(1, y: 2).check(1, 2);
+  A1.b2(1, 2).check(1, 2);
+  A2(1, 2).check(1, 2);
+  A2.b1(1, y: 2).check(1, 2);
+  A2.b2(1, 2).check(1, 2);
+  P1(1, 2).check(1, 2);
+
+  A1(1).check(1, 0);
+  A1.b1(1).check(1, 0);
+  A1.b2(1).check(1, const <int>[0]);
+  A2(1).check(1, 0);
+  A2.b1(1).check(1, 0);
+  A2.b2(1).check(1, const <int>[0]);
+  P1(1).check(1, privateValue);
+
+  const A1(1, 2).check(1, 2);
+  const A1.b1(1, y: 2).check(1, 2);
+  const A1.b2(1, 2).check(1, 2);
+  const A2(1, 2).check(1, 2);
+  const A2.b1(1, y: 2).check(1, 2);
+  const A2.b2(1, 2).check(1, 2);
+  const P1(1, 2).check(1, 2);
+
+  const A1(1).check(1, 0);
+  const A1.b1(1).check(1, 0);
+  const A1.b2(1).check(1, const <int>[0]);
+  const A2(1).check(1, 0);
+  const A2.b1(1).check(1, 0);
+  const A2.b2(1).check(1, const <int>[0]);
+  const P1(1).check(1, privateValue);
+}