simplify constructors, fixes #564

test/codegen/expect/extensions.js

 dart_library.library('extensions', null, /* Imports */[
   'dart_sdk'
 ], function(exports, dart_sdk) {
   'use strict';
   const core = dart_sdk.core;
   const collection = dart_sdk.collection;
   const dart = dart_sdk.dart;
   const dartx = dart_sdk.dartx;
   const extensions = Object.create(null);
   extensions.StringIterable = class StringIterable extends collection.IterableBase$(core.String) {
-    StringIterable() {
+    new() {
       this.iterator = null;
-      super.IterableBase();
+      super.new();
     }
   };
   dart.setSignature(extensions.StringIterable, {});
   dart.defineExtensionMembers(extensions.StringIterable, ['iterator']);
   extensions.main = function() {
     return new extensions.StringIterable();
   };
   dart.fn(extensions.main);
   // Exports:
   exports.extensions = extensions;
 });