Mark the test_postfix_completion test as slow.

Change-Id: I38db4fd08cdec35abf159cc2c9807aede266f1b1
Reviewed-on: https://dart-review.googlesource.com/71187
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Reviewed-by: Steve Messick <messick@google.com>
Commit-Queue: Devon Carew <devoncarew@google.com>
diff --git a/pkg/analysis_server/test/integration/edit/get_postfix_completion_test.dart b/pkg/analysis_server/test/integration/edit/get_postfix_completion_test.dart
index 8e34a3b..d7c5fd3 100644
--- a/pkg/analysis_server/test/integration/edit/get_postfix_completion_test.dart
+++ b/pkg/analysis_server/test/integration/edit/get_postfix_completion_test.dart
@@ -18,6 +18,7 @@
 
 @reflectiveTest
 class GetPostfixCompletionTest extends AbstractAnalysisServerIntegrationTest {
+  @TestTimeout(const Timeout.factor(2))
   test_postfix_completion() async {
     String pathname = sourcePath('test.dart');
     String text = r'''