Format parameter lists with trailing commas like argument lists.

test/whitespace/functions.unit

Index: test/whitespace/functions.unit
diff --git a/test/whitespace/functions.unit b/test/whitespace/functions.unit
index 2e8f4871f2f19f213f0275b22b19dad34bd30e77..206ec2b47fac88a21642fea5a426035e9343b64f 100644
--- a/test/whitespace/functions.unit
+++ b/test/whitespace/functions.unit
@@ -77,10 +77,88 @@ main() sync* {
   var lambda = () sync* {};
 }
 >>> trailing comma in single parameter list
-function(argument   ,   ) {}
+function(parameter   ,   ) {;}
 <<<
-function(argument,) {}
+function(
+  parameter,
+) {
+  ;
+}
 >>> trailing comma in parameter list
-function(argument,argument  ,  ) {}
+function(parameter,parameter  ,  ) {;}
+<<<
+function(
+  parameter,
+  parameter,
+) {
+  ;
+}
+>>> trailing comma in all optional parameter list
+function([parameter,parameter  ,  ]) {;}
+<<<
+function([
+  parameter,
+  parameter,
+]) {
+  ;
+}
+>>> trailing comma in all named parameter list
+function({parameter,parameter  ,  }) {;}
+<<<
+function({
+  parameter,
+  parameter,
+}) {
+  ;
+}
+>>> trailing comma in mixed optional parameter list
+function(parameter,[parameter,parameter  ,  ]) {;}
 <<<
-function(argument, argument,) {}
+function(
+  parameter, [
+  parameter,
+  parameter,
+]) {
+  ;
+}
+>>> trailing comma in mixed named parameter list
+function(parameter,{parameter,parameter  ,  }) {;}
+<<<
+function(
+  parameter, {
+  parameter,
+  parameter,
+}) {
+  ;
+}
+>>> trailing comma with => function containing split
+function(parameter,parameter,) => veryLongBodyThatWraps(argument, argument, argument, argument);
+<<<
+function(
+  parameter,
+  parameter,
+) =>
+    veryLongBodyThatWraps(argument,
+        argument, argument, argument);
+>>> trailing comma with wrap at =>
+function(parameter,parameter,) /* comment */ => "a very very long string";
+<<<
+function(
+  parameter,
+  parameter,
+) /* comment */ =>
+    "a very very long string";
+>>> trailing comma function nested in expression
+main() {
+  someVeryLongFunction(argument, argument, (parameter, parameter,) {;});
+}
+<<< (this looks weird, but it should rare and at least we test it)
+main() {
+  someVeryLongFunction(
+      argument, argument, (
+    parameter,
+    parameter,
+  ) {
+    ;
+  });
+}