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

pkg/analyzer/lib/src/generated/engine.dart

Index: pkg/analyzer/lib/src/generated/engine.dart
diff --git a/pkg/analyzer/lib/src/generated/engine.dart b/pkg/analyzer/lib/src/generated/engine.dart
index 9cd12c7c6c0c394ab7d0c5859ae79012e407636d..bd08147097a909fc32a1a51542ed085f02eff223 100644
--- a/pkg/analyzer/lib/src/generated/engine.dart
+++ b/pkg/analyzer/lib/src/generated/engine.dart
@@ -1087,12 +1087,6 @@ abstract class AnalysisOptions {
   bool get enableTiming;
 
   /**
-   * Return `true` to enable trailing commas in parameter and argument lists
-   * (sdk#26647).
-   */
-  bool get enableTrailingCommas;
-
-  /**
    * A flag indicating whether finer grained dependencies should be used
    * instead of just source level dependencies.
    *
@@ -1347,7 +1341,6 @@ class AnalysisOptionsImpl implements AnalysisOptions {
     enableGenericMethods = options.enableGenericMethods;
     enableSuperMixins = options.enableSuperMixins;
     enableTiming = options.enableTiming;
-    enableTrailingCommas = options.enableTrailingCommas;
     generateImplicitErrors = options.generateImplicitErrors;
     generateSdkErrors = options.generateSdkErrors;
     hint = options.hint;