fix(firestore, web): fix an issue where nested object could be incorrectly decoded from JSObjects

packages/cloud_firestore/cloud_firestore/example/integration_test/field_value_e2e.dart

@@ -189,6 +189,24 @@ void runFieldValueTests() {
         },
         skip: true,
       );
+
+      testWidgets('query should restore nested Timestamp', (_) async {
+        DocumentReference<Map<String, dynamic>> doc =
+            await initializeTest('nested-timestamp');
+        await Future.wait([
+          doc.set({
+            'nested': {
+              'timestamp': Timestamp.fromDate(DateTime(2020)),
+            },
+            'timestamp': Timestamp.fromDate(DateTime(2020)),
+          }),
+        ]);
+
+        final snapshot = await doc.get();
+
+        expect(snapshot.data()!['timestamp'], isA<Timestamp>());
+        expect(snapshot.data()!['nested']['timestamp'], isA<Timestamp>());
+      });
     });
   });
 }

packages/cloud_firestore/cloud_firestore_web/lib/src/interop/utils/utils.dart

@@ -38,8 +38,19 @@ dynamic dartify(dynamic object) {
   }
 
   if (dartObject is Map) {
-    return dartObject
-        .map((key, value) => MapEntry(key as String, dartify(value)));
+    final Map<String, dynamic> map = {};
+    for (final key in dartObject.keys) {
+      final value = dartObject[key];
+      if (value is Map) {
+        map[key as String] =
+            value.map((key, value) => MapEntry(key, dartify(value)));
+      } else if (value is List) {
+        map[key as String] = value.map(dartify).toList();
+      } else {
+        map[key as String] = dartify(value);
+      }
+    }
+    return map;
   }
 
   return dartObject;