Parser support for trailing commas (#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 2312 matching lines @@
     // we allow them to appear in any order so that we can recover faster.
     //
     bool wasInInitializer = _inInitializer;
     _inInitializer = false;
     try {
       Expression argument = parseArgument();
       List<Expression> arguments = <Expression>[argument];
       bool foundNamedArgument = argument is NamedExpression;
       bool generatedError = false;
       while (_optional(TokenType.COMMA)) {
+        if (parseTrailingCommas && _matches(TokenType.CLOSE_PAREN)) {
+          break;
+        }
         argument = parseArgument();
         arguments.add(argument);
         if (argument is NamedExpression) {
           foundNamedArgument = true;
         } else if (foundNamedArgument) {
           if (!generatedError) {
             if (!argument.isSynthetic) {
               // Report the error, once, but allow the arguments to be in any
               // order in the AST.
               _reportErrorForCurrentToken(
@@ skipping 3768 matching lines @@
       if (kind == ParameterKind.REQUIRED && wasOptionalParameter) {
         _reportErrorForNode(
             ParserErrorCode.NORMAL_BEFORE_OPTIONAL_PARAMETERS, parameter);
       }
       //
       // Handle the end of parameter groups.
       //
       // TODO(brianwilkerson) Improve the detection and reporting of missing and
       // mismatched delimiters.
       type = _currentToken.type;
+
+      // Advance past trailing commas as appropriate.
+      if (parseTrailingCommas && type == TokenType.COMMA) {
+        // Only parse commas trailing normal (non-positional/named) params.
+        if (rightSquareBracket == null && rightCurlyBracket == null) {
+          Token next = _peek();
+          if (next.type == TokenType.CLOSE_PAREN ||
+              next.type == TokenType.CLOSE_CURLY_BRACKET ||
+              next.type == TokenType.CLOSE_SQUARE_BRACKET) {
+            _advance();
+            type = _currentToken.type;
+          }
+        }
+      }
+
       if (type == TokenType.CLOSE_SQUARE_BRACKET) {
         rightSquareBracket = getAndAdvance();
         if (leftSquareBracket == null) {
           if (leftCurlyBracket != null) {
             _reportErrorForCurrentToken(
                 ParserErrorCode.WRONG_TERMINATOR_FOR_PARAMETER_GROUP, ["}"]);
             rightCurlyBracket = rightSquareBracket;
             rightSquareBracket = null;
           } else {
             _reportErrorForCurrentToken(
@@ skipping 4329 matching lines @@
    */
   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;
 }