Option to enable analyzer trailing comma support (#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 0f47c443d5cc1a7b51e50d40dd78a1ddd9028347..a25d146bc89f01e4317f8e229219eec1367c127f 100644
--- a/pkg/analyzer/lib/src/task/options.dart
+++ b/pkg/analyzer/lib/src/task/options.dart
@@ -47,6 +47,7 @@ class AnalyzerOptions {
   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 =
@@ -488,6 +489,14 @@ 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 =