fix(database,web): fix an issue that would remove duplicate streams in Debug mode (#13253)

Author: Lyokone

packages/firebase_database/firebase_database_web/lib/src/query_web.dart

@@ -4,6 +4,8 @@
 
 part of '../firebase_database_web.dart';
 
+final Map<String, int> _streamHashCodeMap = {};
+
 /// An implementation of [QueryPlatform] which proxies calls to js objects
 class QueryWeb extends QueryPlatform {
   final DatabasePlatform _database;
@@ -64,8 +66,6 @@ class QueryWeb extends QueryPlatform {
     return instance;
   }
 
-  final Map<String, int> _streamHashCodeMap = {};
-
   @override
   String get path {
     final refPath = Uri.parse(_queryDelegate.ref.toString()).path;