Update lib/src/observable_value.dart

Co-authored-by: Esteban Gehring <esteban.gehring@gmail.com>

lib/src/observable_value.dart

diff --git a/lib/src/observable_value.dart b/lib/src/observable_value.dart
index e78edaa..caf18e1 100644
--- a/lib/src/observable_value.dart
+++ b/lib/src/observable_value.dart
@@ -99,7 +99,7 @@
     // to only contain listeners which are still active.
 
     // Since `add` is asynchronous, this loop will not call any user code.
-    int activeCount = 0;
+    var activeCount = 0;
     for (var i = 0; i < _listeners.length; i++) {
       var controller = _listeners[i];
       if (controller.hasListener) {