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

pkg/analyzer/lib/src/context/context.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.context.context;
 
 import 'dart:async';
 import 'dart:collection';
 
 import 'package:analyzer/dart/ast/ast.dart';
@@ skipping 280 matching lines @@
         ((options is AnalysisOptionsImpl)
             ? this._options.implicitCasts != options.implicitCasts
             : false) ||
         ((options is AnalysisOptionsImpl)
             ? this._options.implicitDynamic != options.implicitDynamic
             : false) ||
         this._options.enableStrictCallChecks !=
             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;
     }
     this._options.analyzeFunctionBodiesPredicate =
         options.analyzeFunctionBodiesPredicate;
     this._options.generateImplicitErrors = options.generateImplicitErrors;
     this._options.generateSdkErrors = options.generateSdkErrors;
     this._options.dart2jsHint = options.dart2jsHint;
     this._options.enableGenericMethods = options.enableGenericMethods;
     this._options.enableAssertMessage = options.enableAssertMessage;
     this._options.enableStrictCallChecks = options.enableStrictCallChecks;
     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;
     this._options.incrementalValidation = options.incrementalValidation;
     this._options.lint = options.lint;
     this._options.preserveComments = options.preserveComments;
     this._options.strongMode = options.strongMode;
     this._options.trackCacheDependencies = options.trackCacheDependencies;
     this._options.finerGrainedInvalidation = options.finerGrainedInvalidation;
     if (options is AnalysisOptionsImpl) {
@@ skipping 1973 matching lines @@
     }
     DartSdk sdk = factory.dartSdk;
     if (sdk == null) {
       throw new IllegalArgumentException(
           "The source factory for an SDK analysis context must have a DartUriResolver");
     }
     return new AnalysisCache(
         <CachePartition>[AnalysisEngine.instance.partitionManager.forSdk(sdk)]);
   }
 }