Default trailing comma support to `true` (#26647).

pkg/analyzer/lib/src/task/dart.dart

 // Copyright (c) 2015, 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 analyzer.src.task.dart;
 
 import 'dart:collection';
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/token.dart';
@@ skipping 4029 matching lines @@
     int modificationTime = getRequiredInput(MODIFICATION_TIME_INPUT_NAME);
     Token tokenStream = getRequiredInput(TOKEN_STREAM_INPUT_NAME);
 
     RecordingErrorListener errorListener = new RecordingErrorListener();
     Parser parser = new Parser(source, errorListener);
     AnalysisOptions options = context.analysisOptions;
     parser.parseAsync = options.enableAsync;
     parser.parseFunctionBodies = options.analyzeFunctionBodiesPredicate(source);
     parser.parseGenericMethods = options.enableGenericMethods;
     parser.parseGenericMethodComments = options.strongMode;
-    parser.parseTrailingCommas = options.enableTrailingCommas;
     CompilationUnit unit = parser.parseCompilationUnit(tokenStream);
     unit.lineInfo = lineInfo;
 
     bool hasNonPartOfDirective = false;
     bool hasPartOfDirective = false;
     HashSet<Source> explicitlyImportedSourceSet = new HashSet<Source>();
     HashSet<Source> exportedSourceSet = new HashSet<Source>();
     HashSet<Source> includedSourceSet = new HashSet<Source>();
     NodeList<Directive> directives = unit.directives;
     int length = directives.length;
@@ skipping 2458 matching lines @@
 
   @override
   bool moveNext() {
     if (_newSources.isEmpty) {
       return false;
     }
     currentTarget = _newSources.removeLast();
     return true;
   }
 }