Optional trailing comma support for list and map literals.

pkg/analyzer_experimental/test/services/formatter_test.dart

 // Copyright (c) 2013, 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.
 
 import 'package:unittest/unittest.dart';
 
 import 'package:analyzer_experimental/src/generated/scanner.dart';
 import 'package:analyzer_experimental/src/services/formatter_impl.dart';
 import 'package:analyzer_experimental/src/services/writer.dart';
 
@@ skipping 653 matching lines @@
         '}'
       );
     });
 
     test('stmt (generics)', () {
       expectStmtFormatsTo(
         'var numbers = <int>[1, 2, (3 + 4)];',
         'var numbers = <int>[1, 2, (3 + 4)];'
       );
     });
+    
+    test('stmt (lists)', () {
+      expectStmtFormatsTo(
+        'var l = [1,2,3,4];',
+        'var l = [1, 2, 3, 4];'
+      );
+      //Dangling ','
+      expectStmtFormatsTo(
+        'var l = [1,];',
+        'var l = [1,];'
+      );
+    });
 
     test('stmt (maps)', () {
       expectStmtFormatsTo(
         'var map = const {"foo": "bar", "fuz": null};',
         'var map = const {"foo": "bar", "fuz": null};'
       );
+      
+      //Dangling ','
+      expectStmtFormatsTo(
+        'var map = {"foo": "bar",};',
+        'var map = {"foo": "bar",};'
+      );
     });
 
     test('stmt (try/catch)', () {
       expectStmtFormatsTo(
         'try {\n'
         'doSomething();\n'
         '} catch (e) {\n'
         'print(e);\n'
         '}',
         'try {\n'
@@ skipping 260 matching lines @@
     new TokenStreamComparator(null, t1, t2).verifyEquals();
 
 expectStreamsNotEqual(Token t1, Token t2) =>
     expect(() => new TokenStreamComparator(null, t1, t2).verifyEquals(),
     throwsA(new isInstanceOf<FormatterException>()));
 
 expectCUFormatsTo(src, expected) => expect(formatCU(src), equals(expected));
 
 expectStmtFormatsTo(src, expected) => expect(formatStatement(src),
     equals(expected));