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;
   const _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;
-      return dart.notNull(a) + dart.notNull(b);
+    clashWithObjectProperty({constructor = null} = Object.create(null)) {
+      return constructor;
+    }
+    clashWithJsReservedName(opts) {
+      let func = opts && 'function' in opts ? opts.function : null;
+      return func;
     }
     get a() {
       return this.x();
     }
     set b(b) {}
     get c() {
       return this[_c];
     }
     set c(c) {
       this[_c] = c;
     }
   }
   dart.setSignature(A, {
     methods: () => ({
       x: [core.int, []],
       y: [core.int, [core.int]],
       z: [core.int, [], [core.num]],
       zz: [core.int, [], [core.int]],
       w: [core.int, [core.int], {b: core.num}],
-      ww: [core.int, [core.int], {b: core.int}]
+      clashWithObjectProperty: [dart.dynamic, [], {constructor: dart.dynamic}],
+      clashWithJsReservedName: [dart.dynamic, [], {function: dart.dynamic}]
     })
   });
   class Bar extends core.Object {
     call(x) {
       return core.print(`hello from ${x}`);
     }
   }
   dart.setSignature(Bar, {
     methods: () => ({call: [dart.dynamic, [dart.dynamic]]})
   });
@@ skipping 14 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;
 });