Use destructuring assignments for named parameters (#180)

test/codegen/expect/methods.js

 dart_library.library('methods', null, /* Imports */[
   "dart/_runtime",
   'dart/core'
 ], /* Lazy imports */[
 ], function(exports, dart, core) {
   'use strict';
   let dartx = dart.dartx;
   let _c = Symbol('_c');
   class A extends core.Object {
     A() {
       this[_c] = 3;
     }
     x() {
       return 42;
     }
     y(a) {
       return a;
     }
     z(b) {
       if (b === void 0)
         b = null;
       return dart.asInt(b);
     }
     zz(b) {
       if (b === void 0)
         b = 0;
       return b;
     }
-    w(a, opts) {
-      let b = opts && 'b' in opts ? opts.b : null;
+    w(a, {b = null} = {}) {
       return dart.asInt(dart.notNull(a) + dart.notNull(b));
     }
-    ww(a, opts) {
-      let b = opts && 'b' in opts ? opts.b : 0;
+    ww(a, {b = 0} = {}) {
       return dart.notNull(a) + dart.notNull(b);
     }
     get a() {
       return this.x();
     }
     set b(b) {}
     get c() {
       return this[_c];
     }
     set c(c) {
@@ skipping 35 matching lines @@
     let c = dart.bind("", dartx.padLeft);
     let r = dart.bind(3.0, dartx.floor);
   }
   dart.fn(test);
   // Exports:
   exports.A = A;
   exports.Bar = Bar;
   exports.Foo = Foo;
   exports.test = test;
 });