Await resolveForCompletion() in performWork().

Change-Id: Ia2ca0a417986f5d2a5dbd19837af342c173ddcc5
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/242483
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Konstantin Shcheglov <scheglov@google.com>
diff --git a/pkg/analyzer/lib/src/dart/analysis/driver.dart b/pkg/analyzer/lib/src/dart/analysis/driver.dart
index 09dea2a..435f11f 100644
--- a/pkg/analyzer/lib/src/dart/analysis/driver.dart
+++ b/pkg/analyzer/lib/src/dart/analysis/driver.dart
@@ -1053,7 +1053,7 @@
     if (_resolveForCompletionRequests.isNotEmpty) {
       final request = _resolveForCompletionRequests.removeLast();
       try {
-        final result = _resolveForCompletion(request);
+        final result = await _resolveForCompletion(request);
         request.completer.complete(result);
       } catch (exception, stackTrace) {
         _reportException(request.path, exception, stackTrace);