diff --git a/sdk/lib/_internal/js_dev_runtime/private/ddc_runtime/errors.dart b/sdk/lib/_internal/js_dev_runtime/private/ddc_runtime/errors.dart
index 36ebdf8a18f..b798adf3399 100644
--- a/sdk/lib/_internal/js_dev_runtime/private/ddc_runtime/errors.dart
+++ b/sdk/lib/_internal/js_dev_runtime/private/ddc_runtime/errors.dart
@@ -102,8 +102,23 @@ String _castErrorMessage(from, to) {
   //     }
   //   }
   // }
-  return "Expected a value of type '${typeName(to)}', "
-      "but got one of type '${typeName(from)}'";
+  var fromName = "'${typeName(from)}'";
+  var toName = "'${typeName(to)}'";
+
+  var toType = to;
+  if (_jsInstanceOf(to, NullableType) || _jsInstanceOf(to, LegacyType)) {
+    toType = to.type;
+  }
+  var fromType = from;
+  if (_jsInstanceOf(from, NullableType) || _jsInstanceOf(from, LegacyType)) {
+    fromType = from.type;
+  }
+
+  if (typeName(fromType) == typeName(toType)) {
+    fromName += ' (in ${getLibraryUri(fromType)})';
+    toName += ' (in ${getLibraryUri(toType)})';
+  }
+  return 'Expected a value of type $toName, but got one of type $fromName';
 }
 
 /// The symbol that references the thrown Dart Object (typically but not
diff --git a/tests/dartdevc/cast_error/lib_a.dart b/tests/dartdevc/cast_error/lib_a.dart
new file mode 100644
index 00000000000..5c6dedf1795
--- /dev/null
+++ b/tests/dartdevc/cast_error/lib_a.dart
@@ -0,0 +1,5 @@
+// Copyright (c) 2021, 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 Animal {}
diff --git a/tests/dartdevc/cast_error/lib_b.dart b/tests/dartdevc/cast_error/lib_b.dart
new file mode 100644
index 00000000000..5c6dedf1795
--- /dev/null
+++ b/tests/dartdevc/cast_error/lib_b.dart
@@ -0,0 +1,5 @@
+// Copyright (c) 2021, 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 Animal {}
diff --git a/tests/dartdevc/cast_error/library_uri_in_message_test.dart b/tests/dartdevc/cast_error/library_uri_in_message_test.dart
new file mode 100644
index 00000000000..42d7b853cd4
--- /dev/null
+++ b/tests/dartdevc/cast_error/library_uri_in_message_test.dart
@@ -0,0 +1,66 @@
+// Copyright (c) 2021, 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 "../utils.dart";
+import "lib_a.dart" as libA;
+import "lib_b.dart" as libB;
+
+void main() {
+  // When the name of the class is the same the error message should include
+  // library URIs for the two classes.
+  var a = libA.Animal();
+  try {
+    a as libB.Animal;
+  } on TypeError catch (error) {
+    var message = error.toString();
+    expectStringContains(
+        "Expected a value of type 'Animal' "
+        "(in org-dartlang-app:/tests/dartdevc/cast_error/lib_b.dart)",
+        message);
+    expectStringContains(
+        "but got one of type 'Animal' "
+        "(in org-dartlang-app:/tests/dartdevc/cast_error/lib_a.dart)",
+        message);
+  }
+  // Verify the libraries are properly ordered.
+  var b = libB.Animal();
+  try {
+    b as libA.Animal;
+  } on TypeError catch (error) {
+    var message = error.toString();
+    expectStringContains(
+        "Expected a value of type 'Animal' "
+        "(in org-dartlang-app:/tests/dartdevc/cast_error/lib_a.dart)",
+        message);
+    expectStringContains(
+        "but got one of type 'Animal' "
+        "(in org-dartlang-app:/tests/dartdevc/cast_error/lib_b.dart)",
+        message);
+  }
+
+  // Shows library URIs when one of the types is nullable.
+  try {
+    b as libA.Animal?;
+  } on TypeError catch (error) {
+    var message = error.toString();
+    expectStringContains(
+        "Expected a value of type 'Animal?' "
+        "(in org-dartlang-app:/tests/dartdevc/cast_error/lib_a.dart)",
+        message);
+    expectStringContains(
+        "but got one of type 'Animal' "
+        "(in org-dartlang-app:/tests/dartdevc/cast_error/lib_b.dart)",
+        message);
+  }
+
+  // URIs are not displayed when the class names are different.
+  try {
+    a as String;
+  } on TypeError catch (error) {
+    var message = error.toString();
+    expectStringContains(
+        "Expected a value of type 'String', but got one of type 'Animal'",
+        message);
+  }
+}
diff --git a/tests/dartdevc_2/cast_error/lib_a.dart b/tests/dartdevc_2/cast_error/lib_a.dart
new file mode 100644
index 00000000000..5c6dedf1795
--- /dev/null
+++ b/tests/dartdevc_2/cast_error/lib_a.dart
@@ -0,0 +1,5 @@
+// Copyright (c) 2021, 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 Animal {}
diff --git a/tests/dartdevc_2/cast_error/lib_b.dart b/tests/dartdevc_2/cast_error/lib_b.dart
new file mode 100644
index 00000000000..5c6dedf1795
--- /dev/null
+++ b/tests/dartdevc_2/cast_error/lib_b.dart
@@ -0,0 +1,5 @@
+// Copyright (c) 2021, 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 Animal {}
diff --git a/tests/dartdevc_2/cast_error/library_uri_in_message_test.dart b/tests/dartdevc_2/cast_error/library_uri_in_message_test.dart
new file mode 100644
index 00000000000..c278d15cf46
--- /dev/null
+++ b/tests/dartdevc_2/cast_error/library_uri_in_message_test.dart
@@ -0,0 +1,53 @@
+// Copyright (c) 2021, 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
+
+import "../utils.dart";
+import "lib_a.dart" as libA;
+import "lib_b.dart" as libB;
+
+void main() {
+  // When the name of the class is the same the error message should include
+  // library URIs for the two classes.
+  var a = libA.Animal();
+  try {
+    a as libB.Animal;
+  } on TypeError catch (error) {
+    var message = error.toString();
+    expectStringContains(
+        "Expected a value of type 'Animal' "
+        "(in org-dartlang-app:/tests/dartdevc_2/cast_error/lib_b.dart)",
+        message);
+    expectStringContains(
+        "but got one of type 'Animal' "
+        "(in org-dartlang-app:/tests/dartdevc_2/cast_error/lib_a.dart)",
+        message);
+  }
+  // Verify the libraries are properly ordered.
+  var b = libB.Animal();
+  try {
+    b as libA.Animal;
+  } on TypeError catch (error) {
+    var message = error.toString();
+    expectStringContains(
+        "Expected a value of type 'Animal' "
+        "(in org-dartlang-app:/tests/dartdevc_2/cast_error/lib_a.dart)",
+        message);
+    expectStringContains(
+        "but got one of type 'Animal' "
+        "(in org-dartlang-app:/tests/dartdevc_2/cast_error/lib_b.dart)",
+        message);
+  }
+
+  // URIs are not displayed when the class names are different.
+  try {
+    a as String;
+  } on TypeError catch (error) {
+    var message = error.toString();
+    expectStringContains(
+        "Expected a value of type 'String', but got one of type 'Animal'",
+        message);
+  }
+}