Await the benchmark reports

Change-Id: I9de7fdbadcf454b1d2069580a0e5c96722b3c7ca
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/287620
Reviewed-by: Alexander Aprelev <aam@google.com>
Commit-Queue: Brian Quinlan <bquinlan@google.com>

benchmarks/FileIOSink/dart/FileIOSink.dart

@@ -101,7 +101,7 @@ class BenchmarkAlternatingSizedAdd extends AsyncBenchmarkBase {
   }
 }
 
-void main() {
+void main() async {
   final benchmarks = [
     BenchmarkManySmallAdds(),
     BenchmarkOneLargeAdd(),
@@ -109,6 +109,6 @@ void main() {
   ];
 
   for (final benchmark in benchmarks) {
-    benchmark.report();
+    await benchmark.report();
   }
 }