Option to enable analyzer trailing comma support (#26647).

pkg/analyzer/lib/src/generated/parser.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 analyzer.src.generated.parser;
 
 import 'dart:collection';
 import "dart:math" as math;
 
 import 'package:analyzer/dart/ast/ast.dart';
@@ skipping 2171 matching lines @@
    */
   bool parseGenericMethods = false;
 
   /**
    * A flag indicating whether to parse generic method comments, of the form
    * `/*=T*/` and `/*<T>*/`.
    */
   bool parseGenericMethodComments = false;
 
   /**
+   * A flag indicating whether the parser is to parse trailing commas in
+   * parameter and argument lists (sdk#26647).
+   */
+  bool parseTrailingCommas = false;
+
+  /**
    * Initialize a newly created parser to parse tokens in the given [_source]
    * and to report any errors that are found to the given [_errorListener].
    */
   Parser(this._source, this._errorListener);
 
   /**
    * Set the token with which the parse is to begin to the given [token].
    */
   void set currentToken(Token token) {
     this._currentToken = token;
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    */
   const ParserErrorCode(String name, String message, [String correction])
       : super(name, message, correction);
 
   @override
   ErrorSeverity get errorSeverity => ErrorSeverity.ERROR;
 
   @override
   ErrorType get type => ErrorType.SYNTACTIC_ERROR;
 }