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

pkg/analysis_server/test/services/completion/completion_target_test.dart

Index: pkg/analysis_server/test/services/completion/completion_target_test.dart
diff --git a/pkg/analysis_server/test/services/completion/completion_target_test.dart b/pkg/analysis_server/test/services/completion/completion_target_test.dart
index f7cca21b8d96b746e60d1a52e2c137b1636da2d7..ce023433eeeb77d456ecfc14d8942908e9f93569 100644
--- a/pkg/analysis_server/test/services/completion/completion_target_test.dart
+++ b/pkg/analysis_server/test/services/completion/completion_target_test.dart
@@ -64,11 +64,7 @@ class CompletionTargetTest extends AbstractContextTest {
   test_ArgumentList_InstanceCreationExpression2() {
     // ArgumentList  InstanceCreationExpression  Block
     addTestSource('main() {new Foo(a,^)}');
-    if (context.analysisOptions.enableTrailingCommas) {
-      assertTarget(')', '(a)', argIndex: 1);
-    } else {
-      assertTarget('', '(a, )', argIndex: 1);
-    }
+    assertTarget(')', '(a)', argIndex: 1);
   }
 
   test_ArgumentList_InstanceCreationExpression_functionArg2() {
@@ -104,11 +100,7 @@ class CompletionTargetTest extends AbstractContextTest {
   test_ArgumentList_MethodInvocation4() {
     // ArgumentList  MethodInvocation  Block
     addTestSource('main() {foo(n,^)}');
-    if (context.analysisOptions.enableTrailingCommas) {
-      assertTarget(')', '(n)', argIndex: 1);
-    } else {
-      assertTarget('', '(n, )', argIndex: 1);
-    }
+    assertTarget(')', '(n)', argIndex: 1);
   }
 
   test_ArgumentList_MethodInvocation_functionArg() {