simplify constructors, fixes #564

test/codegen/expect/js/js.js

 dart_library.library('js', null, /* Imports */[
   'dart_sdk'
 ], function(exports, dart_sdk) {
   'use strict';
   const core = dart_sdk.core;
   const js = dart_sdk.js;
   const dart = dart_sdk.dart;
   const dartx = dart_sdk.dartx;
   const js$ = Object.create(null);
   js$.JS = class JS extends core.Object {
-    JS(name) {
+    new(name) {
       if (name === void 0) name = null;
       this.name = name;
     }
   };
   dart.setSignature(js$.JS, {
-    constructors: () => ({JS: [js$.JS, [], [core.String]]})
+    constructors: () => ({new: [js$.JS, [], [core.String]]})
   });
   js$._Anonymous = class _Anonymous extends core.Object {
-    _Anonymous() {
+    new() {
     }
   };
   dart.setSignature(js$._Anonymous, {
-    constructors: () => ({_Anonymous: [js$._Anonymous, []]})
+    constructors: () => ({new: [js$._Anonymous, []]})
   });
   js$.anonymous = dart.const(new js$._Anonymous());
   js$.allowInteropCaptureThis = js.allowInteropCaptureThis;
   js$.allowInterop = js.allowInterop;
   // Exports:
   exports.js = js$;
 });