Fix `selectionOffset` testing.

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

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library test.services.completion.dart.util;
 
 import 'dart:async';
 
 import 'package:analysis_server/plugin/protocol/protocol.dart' as protocol
     show Element, ElementKind;
@@ skipping 117 matching lines @@
     if (cs == null) {
       failedCompletion('expected $completion $csKind $elemKind', suggestions);
     }
     expect(cs.kind, equals(csKind));
     if (isDeprecated) {
       expect(cs.relevance, equals(DART_RELEVANCE_LOW));
     } else {
       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));
     if (cs.element != null) {
       expect(cs.element.location, isNotNull);
       expect(cs.element.location.file, isNotNull);
       expect(cs.element.location.offset, isNotNull);
       expect(cs.element.location.length, isNotNull);
       expect(cs.element.location.startColumn, isNotNull);
       expect(cs.element.location.startLine, isNotNull);
@@ skipping 487 matching lines @@
 
   @override
   void setUp() {
     super.setUp();
     index = createMemoryIndex();
     searchEngine =
         new SearchEngineImpl(index, (_) => new AstProviderForContext(context));
     contributor = createContributor();
   }
 }