Fix for extra diagnostics when now all files created at the start.

Change-Id: Icda792b55a1381294c4e3490694fb99ee2c4ca3f
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/302446
Commit-Queue: Konstantin Shcheglov <scheglov@google.com>
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>

pkg/analysis_server/test/client/completion_driver_test.dart

@@ -72,10 +72,15 @@ abstract class AbstractCompletionDriverTest
  Future<List<CompletionSuggestion>> addTestFile(String content,
  {int? offset}) async {
  driver.addTestFile(content, offset: offset);
- await getSuggestions();
+
+ // Wait after adding the test file, this might affect diagnostics.
+ await pumpEventQueue(times: 1000);
+
  // For sanity, ensure that there are no errors recorded for project files
  // since that may lead to unexpected results.
  _assertNoErrorsInProjectFiles();
+
+ await getSuggestions();
  return suggestions;
  }