simplify constructors, fixes #564

test/codegen/expect/opassign.js

 dart_library.library('opassign', null, /* Imports */[
   'dart_sdk'
 ], function(exports, dart_sdk) {
   'use strict';
   const core = dart_sdk.core;
   const dart = dart_sdk.dart;
   const dartx = dart_sdk.dartx;
   const opassign = Object.create(null);
   dart.copyProperties(opassign, {
     get index() {
       core.print('called "index" getter');
       return 0;
     }
   });
   dart.defineLazy(opassign, {
     get _foo() {
       return new opassign.Foo();
     }
   });
   dart.copyProperties(opassign, {
     get foo() {
       core.print('called "foo" getter');
       return opassign._foo;
     }
   });
   opassign.Foo = class Foo extends core.Object {
-    Foo() {
+    new() {
       this.x = 100;
     }
   };
   opassign.main = function() {
     let f = dart.map([0, 40]);
     core.print('should only call "index" 2 times:');
     let i = dart.as(opassign.index, core.int);
     f[dartx.set](i, dart.notNull(f[dartx.get](i)) + 1);
     opassign.forcePostfix((() => {
       let i = dart.as(opassign.index, core.int), x = f[dartx.get](i);
@@ skipping 12 matching lines @@
     let i$ = dart.as(opassign.index, core.int);
     f[dartx.set](i$, dart.notNull(f[dartx.get](i$)) + dart.notNull(f[dartx.get](opassign.index)));
   };
   dart.fn(opassign.main);
   opassign.forcePostfix = function(x) {
   };
   dart.fn(opassign.forcePostfix);
   // Exports:
   exports.opassign = opassign;
 });