Use destructuring assignments for named parameters (#180)

test/codegen/expect/constructors.js

 dart_library.library('constructors', null, /* Imports */[
   "dart/_runtime",
   'dart/core'
 ], /* Lazy imports */[
 ], function(exports, dart, core) {
   'use strict';
   let dartx = dart.dartx;
   class A extends core.Object {}
   class B extends core.Object {
     B() {
@@ skipping 51 matching lines @@
   class G extends core.Object {
     G(p1) {
       if (p1 === void 0)
         p1 = null;
     }
   }
   dart.setSignature(G, {
     constructors: () => ({G: [G, [], [core.String]]})
   });
   class H extends core.Object {
-    H(opts) {
-      let p1 = opts && 'p1' in opts ? opts.p1 : null;
+    H({p1 = null} = {}) {
     }
   }
   dart.setSignature(H, {
     constructors: () => ({H: [H, [], {p1: core.String}]})
   });
   class I extends core.Object {
     I() {
       this.name = 'default';
     }
     named(name) {
@@ skipping 126 matching lines @@
   exports.I = I;
   exports.J = J;
   exports.K = K;
   exports.L = L;
   exports.M = M;
   exports.N = N;
   exports.P = P;
   exports.Q$ = Q$;
   exports.Q = Q;
 });