Drop use of pkg:collection whereNotNull() (dart-archive/async#278)

Exists in SDK as of v3.0

Author: kevmoo

pkgs/async/lib/src/cancelable_operation.dart

@@ -4,8 +4,6 @@
 
 import 'dart:async';
 
-import 'package:collection/collection.dart';
-
 /// An asynchronous operation that can be cancelled.
 ///
 /// The value of this operation is exposed as [value]. When this operation is
@@ -521,7 +519,7 @@ class CancelableCompleter<T> {
     final isFuture = toReturn is Future;
     final cancelFutures = <Future<Object?>>[
       if (isFuture) toReturn,
-      ...?_cancelForwarders?.map(_forward).whereNotNull()
+      ...?_cancelForwarders?.map(_forward).nonNulls
     ];
     final results = (isFuture && cancelFutures.length == 1)
         ? [await toReturn]

pkgs/async/lib/src/stream_group.dart

@@ -4,8 +4,6 @@
 
 import 'dart:async';
 
-import 'package:collection/collection.dart';
-
 /// A collection of streams whose events are unified and sent through a central
 /// stream.
 ///
@@ -239,7 +237,7 @@ class StreamGroup<T> implements Sink<Stream<T>> {
             return null;
           }
         })
-        .whereNotNull()
+        .nonNulls
         .toList();
 
     _subscriptions.clear();

pkgs/async/lib/src/stream_queue.dart

@@ -5,7 +5,7 @@
 import 'dart:async';
 import 'dart:collection';
 
-import 'package:collection/collection.dart';
+import 'package:collection/collection.dart' show QueueList;
 
 import 'cancelable_operation.dart';
 import 'result/result.dart';