Give better error message for describeEnum (#61581)

packages/flutter/lib/src/foundation/diagnostics.dart

@@ -3032,7 +3032,10 @@ String describeIdentity(Object? object) => '${objectRuntimeType(object, '<optimi
 String describeEnum(Object enumEntry) {
   final String description = enumEntry.toString();
   final int indexOfDot = description.indexOf('.');
-  assert(indexOfDot != -1 && indexOfDot < description.length - 1);
+  assert(
+    indexOfDot != -1 && indexOfDot < description.length - 1,
+    'The provided object "$enumEntry" is not an enum.',
+  );
   return description.substring(indexOfDot + 1);
 }
 

packages/flutter/test/foundation/diagnostics_test.dart

@@ -903,6 +903,14 @@ void main() {
     expect(describeEnum(ExampleEnum.hello), equals('hello'));
     expect(describeEnum(ExampleEnum.world), equals('world'));
     expect(describeEnum(ExampleEnum.deferToChild), equals('deferToChild'));
+    expect(
+      () => describeEnum('Hello World'),
+      throwsA(isAssertionError.having(
+        (AssertionError e) => e.message,
+        'message',
+        'The provided object "Hello World" is not an enum.'),
+      ),
+    );
   });
 
   test('string property test', () {