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

pkg/analyzer/lib/src/task/options.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.options;
 
 import 'dart:collection';
 
 import 'package:analyzer/analyzer.dart';
 import 'package:analyzer/plugin/options.dart';
@@ skipping 29 matching lines @@
         AnalysisContext context, Map<String, Object> options) =>
     _processor.configure(context, options);
 
 /// `analyzer` analysis options constants.
 class AnalyzerOptions {
   static const String analyzer = 'analyzer';
   static const String enableAsync = 'enableAsync';
   static const String enableGenericMethods = 'enableGenericMethods';
   static const String enableStrictCallChecks = 'enableStrictCallChecks';
   static const String enableSuperMixins = 'enableSuperMixins';
+  static const String enableTrailingCommas = 'enableTrailingCommas';
 
   /// This option is deprecated.
   static const String enableConditionalDirectives =
       "enableConditionalDirectives";
   static const String errors = 'errors';
   static const String exclude = 'exclude';
   static const String language = 'language';
   static const String plugins = 'plugins';
   static const String strong_mode = 'strong-mode';
 
@@ skipping 421 matching lines @@
       }
     }
     if (feature == AnalyzerOptions.enableSuperMixins) {
       if (isTrue(value)) {
         AnalysisOptionsImpl options =
             new AnalysisOptionsImpl.from(context.analysisOptions);
         options.enableSuperMixins = true;
         context.analysisOptions = options;
       }
     }
+    if (feature == AnalyzerOptions.enableTrailingCommas) {
+      if (isTrue(value)) {
+        AnalysisOptionsImpl options =
+            new AnalysisOptionsImpl.from(context.analysisOptions);
+        options.enableTrailingCommas = true;
+        context.analysisOptions = options;
+      }
+    }
     if (feature == AnalyzerOptions.enableGenericMethods) {
       if (isTrue(value)) {
         AnalysisOptionsImpl options =
             new AnalysisOptionsImpl.from(context.analysisOptions);
         options.enableGenericMethods = true;
         context.analysisOptions = options;
       }
     }
   }
 
@@ skipping 49 matching lines @@
           String feature = key.value?.toString();
           _applyLanguageOption(options, feature, value.value);
         }
       });
     } else if (configs is Map) {
       configs
           .forEach((key, value) => _applyLanguageOption(options, key, value));
     }
   }
 }