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

pkg/analyzer/lib/src/context/context.dart

Index: pkg/analyzer/lib/src/context/context.dart
diff --git a/pkg/analyzer/lib/src/context/context.dart b/pkg/analyzer/lib/src/context/context.dart
index 7ef0108f626702d81591efa686fecfcf9643cdb4..117204a43fc96a7da6b87c10d34fd81a3f978a9c 100644
--- a/pkg/analyzer/lib/src/context/context.dart
+++ b/pkg/analyzer/lib/src/context/context.dart
@@ -298,8 +298,7 @@ class AnalysisContextImpl implements InternalAnalysisContext {
             options.enableStrictCallChecks ||
         this._options.enableGenericMethods != options.enableGenericMethods ||
         this._options.enableAsync != options.enableAsync ||
-        this._options.enableSuperMixins != options.enableSuperMixins ||
-        this._options.enableTrailingCommas != options.enableTrailingCommas;
+        this._options.enableSuperMixins != options.enableSuperMixins;
     int cacheSize = options.cacheSize;
     if (this._options.cacheSize != cacheSize) {
       this._options.cacheSize = cacheSize;
@@ -315,7 +314,6 @@ class AnalysisContextImpl implements InternalAnalysisContext {
     this._options.enableAsync = options.enableAsync;
     this._options.enableSuperMixins = options.enableSuperMixins;
     this._options.enableTiming = options.enableTiming;
-    this._options.enableTrailingCommas = options.enableTrailingCommas;
     this._options.hint = options.hint;
     this._options.incremental = options.incremental;
     this._options.incrementalApi = options.incrementalApi;