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

pkg/analyzer/lib/src/task/options.dart

Index: pkg/analyzer/lib/src/task/options.dart
diff --git a/pkg/analyzer/lib/src/task/options.dart b/pkg/analyzer/lib/src/task/options.dart
index a101147344ffc772f1c17454c00ca3c055ff9c1d..6cf558582f4f160f0c9a672b364fc859018f53d6 100644
--- a/pkg/analyzer/lib/src/task/options.dart
+++ b/pkg/analyzer/lib/src/task/options.dart
@@ -49,7 +49,6 @@ class AnalyzerOptions {
   static const String enableGenericMethods = 'enableGenericMethods';
   static const String enableStrictCallChecks = 'enableStrictCallChecks';
   static const String enableSuperMixins = 'enableSuperMixins';
-  static const String enableTrailingCommas = 'enableTrailingCommas';
 
   static const String errors = 'errors';
   static const String exclude = 'exclude';
@@ -88,8 +87,7 @@ class AnalyzerOptions {
     enableAsync,
     enableGenericMethods,
     enableStrictCallChecks,
-    enableSuperMixins,
-    enableTrailingCommas
+    enableSuperMixins
   ];
 }
 
@@ -504,14 +502,6 @@ class _OptionsProcessor {
         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 =