fix: analyzer when using Dart 2.16.0 (#663)

* fix: analyzer when using Dart 2.16.0

* update rxdart-test.yml

Author: hoc081098

.github/workflows/rxdart-test.yml

@@ -9,14 +9,14 @@ on:
     paths-ignore: [ '**.md' ]
   schedule:
     # Runs at 00:00 UTC on the 1, 8, 15, 22 and 29th of every month.
-    - cron:  '0 0 */7 * *'
+    - cron: '0 0 */7 * *'
 jobs:
   analyze-and-test:
     runs-on: ubuntu-latest
 
     strategy:
       matrix:
-        sdk: [ stable, beta, dev, 2.12.0 ]
+        sdk: [ beta, stable, 2.15.0, 2.12.0 ]
 
     steps:
       - uses: actions/checkout@v2

lib/src/transformers/group_by.dart

@@ -72,6 +72,7 @@ class _GroupByStreamSink<T, K> extends ForwardingSink<T, GroupedStream<T, K>> {
       subscriptions = null;
       return future;
     }
+    return null;
   }
 
   @override

lib/src/utils/composite_subscription.dart

@@ -53,11 +53,10 @@ class CompositeSubscription implements StreamSubscription<Never> {
   Future<void>? remove(
     StreamSubscription<dynamic> subscription, {
     bool shouldCancel = true,
-  }) {
-    if (_subscriptionsList.remove(subscription) && shouldCancel) {
-      return subscription.cancel();
-    }
-  }
+  }) =>
+      _subscriptionsList.remove(subscription) && shouldCancel
+          ? subscription.cancel()
+          : null;
 
   /// Cancels all subscriptions added to this composite. Clears subscriptions collection.
   ///