Fix `selectionOffset` testing.

pkg/analysis_server/test/services/completion/dart/completion_contributor_util.dart

Index: pkg/analysis_server/test/services/completion/dart/completion_contributor_util.dart
diff --git a/pkg/analysis_server/test/services/completion/dart/completion_contributor_util.dart b/pkg/analysis_server/test/services/completion/dart/completion_contributor_util.dart
index 1acc5b94199046a676f05d8a433e277ef5b2cc5e..9f75662be73aae9b4275b211d549f125820664db 100644
--- a/pkg/analysis_server/test/services/completion/dart/completion_contributor_util.dart
+++ b/pkg/analysis_server/test/services/completion/dart/completion_contributor_util.dart
@@ -135,11 +135,7 @@ abstract class DartCompletionContributorTest extends AbstractContextTest {
       expect(cs.relevance, equals(relevance), reason: completion);
     }
     expect(cs.importUri, importUri);
-    //TODO(pq): fix offset bug for trailing commas
-    // and replace w/ expect(cs.selectionOffset, equals(selectionOffset ?? completion.length));
-    if (selectionOffset != null) {
-      expect(cs.selectionOffset, equals(selectionOffset));
-    }
+    expect(cs.selectionOffset, equals(selectionOffset ?? completion.length));
     expect(cs.selectionLength, equals(0));
     expect(cs.isDeprecated, equals(isDeprecated));
     expect(cs.isPotential, equals(isPotential));