Delinting future awaits round 2 (#10790)

* round 2

* deal with null futures

* review

* review

* review

* review

packages/flutter_tools/lib/src/base/utils.dart

diff --git a/packages/flutter_tools/lib/src/base/utils.dart b/packages/flutter_tools/lib/src/base/utils.dart
index fddf40c..af7b8b8 100644
--- a/packages/flutter_tools/lib/src/base/utils.dart
+++ b/packages/flutter_tools/lib/src/base/utils.dart
@@ -256,3 +256,14 @@
     _timer = null;
   }
 }
+
+/// Returns a [Future] that completes when all given [Future]s complete.
+///
+/// Uses [Future.wait] but removes null elements from the provided
+/// `futures` iterable first.
+///
+/// The returned [Future<List>] will be shorter than the given `futures` if
+/// it contains nulls.
+Future<List<T>> waitGroup<T>(Iterable<Future<T>> futures) {
+  return Future.wait<T>(futures.where((Future<T> future) => future != null));
+}