fixes #168, dart:js implementation with a test

test/codegen/expect/sunflower/dom.js

 dart_library.library('dom', window, /* Imports */[
   "dart_runtime/dart",
   'dart/core'
 ], /* Lazy imports */[
 ], function(exports, dart, core) {
   'use strict';
   let dartx = dart.dartx;
   class JsName extends core.Object {
     JsName(opts) {
       let name = opts && 'name' in opts ? opts.name : null;
       this.name = name;
     }
   }
   dart.setSignature(JsName, {
     constructors: () => ({JsName: [JsName, [], {name: core.String}]})
   });
   class Overload extends core.Object {
     Overload() {
     }
   }
   dart.setSignature(Overload, {
     constructors: () => ({Overload: [Overload, []]})
   });
   let overload = dart.const(new Overload());
+  class CustomEvent extends core.Object {}
+  dart.setSignature(CustomEvent, {
+    constructors: () => ({CustomEvent: [CustomEvent, [core.String], {detail: dart.dynamic, bubbles: dart.dynamic, cancelable: dart.dynamic}]})
+  });
+  class HTMLCollection extends core.Object {}
+  dart.setSignature(HTMLCollection, {
+    methods: () => ({get: [Element, [core.num]]})
+  });
   let EventListener = dart.typedef('EventListener', () => dart.functionType(dart.void, [Event]));
   let InputElement = HTMLInputElement;
   let CanvasElement = HTMLCanvasElement;
+  let DivElement = HTMLDivElement;
+  let ScriptElement = HTMLScriptElement;
   class RenderingContext extends core.Object {}
   class CanvasDrawingStyles extends core.Object {
     CanvasDrawingStyles() {
       this.lineWidth = null;
       this.lineCap = null;
       this.lineJoin = null;
       this.miterLimit = null;
       this.lineDashOffset = null;
       this.font = null;
       this.textAlign = null;
       this.textBaseline = null;
     }
   }
   class CanvasPathMethods extends core.Object {}
   // Exports:
   exports.JsName = JsName;
   exports.Overload = Overload;
   exports.overload = overload;
+  exports.CustomEvent = CustomEvent;
+  exports.HTMLCollection = HTMLCollection;
   exports.EventListener = EventListener;
   exports.InputElement = InputElement;
   exports.CanvasElement = CanvasElement;
+  exports.DivElement = DivElement;
+  exports.ScriptElement = ScriptElement;
   exports.RenderingContext = RenderingContext;
   exports.CanvasDrawingStyles = CanvasDrawingStyles;
   exports.CanvasPathMethods = CanvasPathMethods;
 });