Small cleanups around ExtensionRegistry.reparseMessage (#322)

diff --git a/protobuf/lib/src/protobuf/extension_registry.dart b/protobuf/lib/src/protobuf/extension_registry.dart
index 0017daa..fd04cda 100644
--- a/protobuf/lib/src/protobuf/extension_registry.dart
+++ b/protobuf/lib/src/protobuf/extension_registry.dart
@@ -146,10 +146,10 @@
           }
         }
       }
-    } else if (field.isMapField) {
+    } else if (field is MapFieldInfo) {
       final messageMap = message._fieldSet._values[field.index];
       if (messageMap == null) return;
-      if (_isGroupOrMessage((field as MapFieldInfo).valueFieldType)) {
+      if (_isGroupOrMessage(field.valueFieldType)) {
         for (var key in messageMap.keys) {
           final GeneratedMessage value = messageMap[key];
           final GeneratedMessage reparsedValue =
diff --git a/protoc_plugin/test/extension_test.dart b/protoc_plugin/test/extension_test.dart
index 07fce89..0bbdb59 100644
--- a/protoc_plugin/test/extension_test.dart
+++ b/protoc_plugin/test/extension_test.dart
@@ -547,8 +547,7 @@
         isTrue);
     expect(
         identical(withUnknownFields.innerMap[1], reparsed.innerMap[1]), isTrue);
-    expect(withUnknownFields.stringMap.length == reparsed.stringMap.length,
-        isTrue);
-    expect(withUnknownFields.stringMap[0] == reparsed.stringMap[0], isTrue);
+    expect(withUnknownFields.stringMap.length, reparsed.stringMap.length);
+    expect(withUnknownFields.stringMap[0], reparsed.stringMap[0]);
   });
 }