Format parameter lists with trailing commas like argument lists.

test/comments/functions.unit

 40 columns                              |
 >>> indented line comment (dartbug.com/16383)
 main() {
   // comment
 }
 <<<
 main() {
   // comment
 }
 >>> line comment on opening line
@@ skipping 108 matching lines @@
 >>> remove blank line before beginning of body
 main() {
 
 
 
   // comment
 }
 <<<
 main() {
   // comment
+}
+>>> comment before "]" with trailing comma
+function([parameter,/* c */]) {;}
+<<<
+function([
+  parameter,
+  /* c */
+]) {
+  ;
+}
+>>> comment before ")" with trailing comma
+function([parameter,]/* c */) {;}
+<<<
+function([
+  parameter,
+] /* c */) {
+  ;
 }