Optional trailing comma support for list and map literals.

pkg/analyzer_experimental/test/services/formatter_test.dart

Index: pkg/analyzer_experimental/test/services/formatter_test.dart
===================================================================
--- pkg/analyzer_experimental/test/services/formatter_test.dart	(revision 27343)
+++ pkg/analyzer_experimental/test/services/formatter_test.dart	(working copy)
@@ -671,12 +671,30 @@
         '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)', () {