Always split collections and argument lists with trailing commas.

test/splitting/maps.stmt

Index: test/splitting/maps.stmt
diff --git a/test/splitting/maps.stmt b/test/splitting/maps.stmt
index 791eaa7d7718a03bd485fe226fd61eccc02e141d..31da5ab7e2345a2821bfa2bc0e9b07b478591d97 100644
--- a/test/splitting/maps.stmt
+++ b/test/splitting/maps.stmt
@@ -74,11 +74,13 @@ var map = const {
   "foo": "bar",
   "fuz": null
 };
->>> dangling comma
+>>> trailing comma forces split
 var map = {"foo": "bar" , };
 <<<
-var map = {"foo": "bar",};
->>> dangling comma multiline
+var map = {
+  "foo": "bar",
+};
+>>> trailing comma multiline
 var map = {"foo": "bar", "fuzzy": null , };
 <<<
 var map = {