Option to enable analyzer trailing comma support (#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 2b7aa9fcf105d66163db0565172353b5d9eda481..a49b8ead214852ea0275c6931d8d04f0ab8b9b62 100644
--- a/pkg/analyzer/lib/src/generated/parser.dart
+++ b/pkg/analyzer/lib/src/generated/parser.dart
@@ -2189,6 +2189,12 @@ 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].
    */