Use destructuring assignments for named parameters (#180)

test/codegen/expect/closure.js

 dart_library.library('closure', null, /* Imports */[
   "dart/_runtime",
   'dart/core',
   'dart/js'
 ], /* Lazy imports */[
 ], function(exports, dart, core, js) {
   'use strict';
   let dartx = dart.dartx;
   /** @typedef {function({i: (?number|undefined)}=)} */
   const Callback = dart.typedef('Callback', () => dart.functionType(dart.void, [], {i: core.int}));
@@ skipping 47 matching lines @@
       optional_params(a, b, c) {
         if (b === void 0)
           b = null;
         if (c === void 0)
           c = null;
       }
       /**
        * @param {?} a
        * @param {{b: (?|undefined), c: (?|undefined)}=} opts
        */
-      static named_params(a, opts) {
-        let b = opts && 'b' in opts ? opts.b : null;
-        let c = opts && 'c' in opts ? opts.c : null;
-      }
+      static named_params(a, {b = null, c = null} = {}) {}
       nullary_method() {}
       /**
        * @param {function(?, ?=):?number} f
        * @param {function(?, {y: (string|undefined), z: (?|undefined)}=):?} g
        * @param {Callback} cb
        */
       function_params(f, g, cb) {
         dart.as(f, dart.functionType(core.int, [dart.dynamic], [dart.dynamic]));
         dart.as(g, dart.functionType(dart.dynamic, [dart.dynamic], {y: core.String, z: dart.dynamic}));
         cb({i: this.i});
@@ skipping 58 matching lines @@
   exports.some_top_level_var = "abc";
   // Exports:
   exports.Callback = Callback;
   exports.Foo$ = Foo$;
   exports.Foo = Foo;
   exports.Bar = Bar;
   exports.Baz = Baz;
   exports.main = main;
   exports.some_top_level_constant = some_top_level_constant;
 });