Fix getProperties invocation passing length for all objects. (#2480)

Co-authored-by: Nate Biggs <natebiggs@google.com>

dwds/lib/src/debugging/instance.dart

@@ -268,7 +268,9 @@ class InstanceHelper extends Domain {
       objectId,
       offset: offset,
       count: count,
-      length: metaData.length,
+      length: metaData.kind != InstanceKind.kPlainInstance
+          ? metaData.length
+          : null,
     );
 
     final dartProperties = await _dartFieldsFor(properties, remoteObject);