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

pkg/analyzer/lib/src/generated/parser.dart

Index: pkg/analyzer/lib/src/generated/parser.dart
diff --git a/pkg/analyzer/lib/src/generated/parser.dart b/pkg/analyzer/lib/src/generated/parser.dart
index f63bb70615a521d647311d2c4335ae9acddaa1a5..fcf9b0132a2d4265d7527c05f679a4c09e57fd81 100644
--- a/pkg/analyzer/lib/src/generated/parser.dart
+++ b/pkg/analyzer/lib/src/generated/parser.dart
@@ -2189,12 +2189,6 @@ class Parser {
   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].
    */
@@ -2330,7 +2324,7 @@ class Parser {
       bool foundNamedArgument = argument is NamedExpression;
       bool generatedError = false;
       while (_optional(TokenType.COMMA)) {
-        if (parseTrailingCommas && _matches(TokenType.CLOSE_PAREN)) {
+        if (_matches(TokenType.CLOSE_PAREN)) {
           break;
         }
         argument = parseArgument();
@@ -6146,7 +6140,7 @@ class Parser {
       type = _currentToken.type;
 
       // Advance past trailing commas as appropriate.
-      if (parseTrailingCommas && type == TokenType.COMMA) {
+      if (type == TokenType.COMMA) {
         // Only parse commas trailing normal (non-positional/named) params.
         if (rightSquareBracket == null && rightCurlyBracket == null) {
           Token next = _peek();