simplify generation of functions

test/codegen/expect/destructuring.js

 dart_library.library('destructuring', null, /* Imports */[
   'dart/_runtime',
   'dart/core'
 ], /* Lazy imports */[
 ], function(exports, dart, core) {
   'use strict';
   let dartx = dart.dartx;
   function f(a, b, c = 1) {
     f(a, b, c);
   }
   dart.fn(f, dart.dynamic, [core.int, dart.dynamic], [dart.dynamic]);
-  function f_sync(a, b, c = 1) {
+  function f_sync(a, b, c) {
     return dart.syncStar(function*(a, b, c = 1) {
     }, dart.dynamic, a, b, c);
   }
   dart.fn(f_sync, dart.dynamic, [core.int, dart.dynamic], [dart.dynamic]);
-  function f_async(a, b, c = 1) {
+  function f_async(a, b, c) {
     return dart.asyncStar(function*(stream, a, b, c = 1) {
     }, dart.dynamic, a, b, c);
   }
   dart.fn(f_async, dart.dynamic, [core.int, dart.dynamic], [dart.dynamic]);
   function g(a, b, {c = 1} = {}) {
     f(a, b, c);
   }
   dart.fn(g, dart.dynamic, [core.int, dart.dynamic], {c: dart.dynamic});
-  function g_sync(a, b, {c = 1} = {}) {
+  function g_sync(a, b, opts) {
     return dart.syncStar(function*(a, b, {c = 1} = {}) {
     }, dart.dynamic, a, b, opts);
   }
   dart.fn(g_sync, dart.dynamic, [core.int, dart.dynamic], {c: dart.dynamic});
-  function g_async(a, b, {c = 1} = {}) {
+  function g_async(a, b, opts) {
     return dart.asyncStar(function*(stream, a, b, {c = 1} = {}) {
     }, dart.dynamic, a, b, opts);
   }
   dart.fn(g_async, dart.dynamic, [core.int, dart.dynamic], {c: dart.dynamic});
   function r(a, ...others) {
     r(a, ...others);
   }
   dart.fn(r, dart.dynamic, [core.int, dart.dynamic]);
   function r_sync(a, ...others) {
     return dart.syncStar(function*(a, ...others) {
@@ skipping 29 matching lines @@
   exports.g_sync = g_sync;
   exports.g_async = g_async;
   exports.r = r;
   exports.r_sync = r_sync;
   exports.r_async = r_async;
   exports.invalid_names1 = invalid_names1;
   exports.invalid_names2 = invalid_names2;
   exports.invalid_names3 = invalid_names3;
   exports.names_clashing_with_object_props = names_clashing_with_object_props;
 });