fix code completion when enableTrailingCommas is true

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

 // Copyright (c) 2014, 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 test.services.completion.target;
 
 import 'package:analysis_server/src/provisional/completion/dart/completion_target.dart';
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/src/generated/source.dart';
 import 'package:test_reflective_loader/test_reflective_loader.dart';
@@ skipping 43 matching lines @@
     assertCommon();
     expect(target.isFunctionalArgument(), isFunctionalArgument);
   }
 
   test_ArgumentList_InstanceCreationExpression() {
     // ArgumentList  InstanceCreationExpression  Block
     addTestSource('main() {new Foo(^)}');
     assertTarget(')', '()', argIndex: 0);
   }
 
+  test_ArgumentList_InstanceCreationExpression2() {
+    // ArgumentList  InstanceCreationExpression  Block
+    addTestSource('main() {new Foo(a,^)}');
+    if (context.analysisOptions.enableTrailingCommas) {
+      assertTarget(')', '(a)', argIndex: 1);
+    } else {
+      assertTarget('', '(a, )', argIndex: 1);
+    }
+  }
+
   test_ArgumentList_InstanceCreationExpression_functionArg2() {
     // ArgumentList  InstanceCreationExpression  Block
     addTestSource('main() {new B(^)} class B{B(f()){}}');
     assertTarget(')', '()', argIndex: 0, isFunctionalArgument: true);
   }
 
   test_ArgumentList_MethodInvocation() {
     // ArgumentList  MethodInvocation  Block
     addTestSource('main() {foo(^)}');
     assertTarget(')', '()', argIndex: 0);
   }
 
   test_ArgumentList_MethodInvocation2() {
     // ArgumentList  MethodInvocation  Block
     addTestSource('main() {foo(^n)}');
     assertTarget('n', '(n)', argIndex: 0);
   }
 
   test_ArgumentList_MethodInvocation3() {
     // ArgumentList  MethodInvocation  Block
     addTestSource('main() {foo(n^)}');
     assertTarget('n', '(n)', argIndex: 0);
   }
 
+  test_ArgumentList_MethodInvocation3a() {
+    // ArgumentList  MethodInvocation  Block
+    addTestSource('main() {foo((n)^)}');
+    assertTarget(')', '((n))', argIndex: 0);
+  }
+
   test_ArgumentList_MethodInvocation4() {
     // ArgumentList  MethodInvocation  Block
     addTestSource('main() {foo(n,^)}');
-    assertTarget('', '(n, )', argIndex: 1);
+    if (context.analysisOptions.enableTrailingCommas) {
+      assertTarget(')', '(n)', argIndex: 1);
+    } else {
+      assertTarget('', '(n, )', argIndex: 1);
+    }
   }
 
   test_ArgumentList_MethodInvocation_functionArg() {
     // ArgumentList  MethodInvocation  Block
     addTestSource('main() {foo(^)} foo(f()) {}');
     assertTarget(')', '()', argIndex: 0, isFunctionalArgument: true);
   }
 
   test_ArgumentList_MethodInvocation_functionArg2() {
     // ArgumentList  MethodInvocation  Block
@@ skipping 448 matching lines @@
     addTestSource('main() {int b^ = 1;}');
     assertTarget('b = 1', 'int b = 1');
   }
 
   test_VariableDeclaration_lhs_identifier_before() {
     // VariableDeclaration  VariableDeclarationList
     addTestSource('main() {int ^b = 1;}');
     assertTarget('b = 1', 'int b = 1');
   }
 }