use js_ast instead of strings to generate JS

test/codegen/expect/dom/dom.js

Index: test/codegen/expect/dom/dom.js
diff --git a/test/codegen/expect/dom/dom.js b/test/codegen/expect/dom/dom.js
index ea98603175b797b77f6a05c80ca3d79cea2c6573..86ffb73f0d5f4940fce31816e99b28c47c3f65c0 100644
--- a/test/codegen/expect/dom/dom.js
+++ b/test/codegen/expect/dom/dom.js
@@ -1,50 +1,41 @@
 var dom;
-(function (dom) {
+(function(dom) {
   'use strict';
   class JsType extends dart.Object {
     JsType(opt$) {
-      let name = opt$.name === undefined ? null : opt$.name;
+      let name = opt$.name === void 0 ? null : opt$.name;
       this.name = name;
     }
   }
-
   class JsGlobal extends dart.Object {
     JsGlobal() {
     }
   }
-
   class Overload extends dart.Object {
     Overload() {
     }
   }
-
   let overload = new Overload();
   dom.document = null;
   class Document extends dart.Object {
   }
-
   class Element extends dart.Object {
     Element() {
       this.textContent = null;
     }
   }
-
   class Event extends dart.Object {
   }
-
   class InputElement extends Element {
     InputElement() {
       this.value = null;
       super.Element();
     }
   }
-
   class CanvasElement extends Element {
   }
-
   class RenderingContext extends dart.Object {
   }
-
   class CanvasRenderingContext2D extends dart.Object {
     CanvasRenderingContext2D() {
       this.globalAlpha = null;
@@ -57,7 +48,6 @@ var dom;
       this.shadowColor = null;
     }
   }
-
   class CanvasDrawingStyles extends dart.Object {
     CanvasDrawingStyles() {
       this.lineWidth = null;
@@ -70,22 +60,16 @@ var dom;
       this.textBaseline = null;
     }
   }
-
   class CanvasPathMethods extends dart.Object {
   }
-
   class CanvasGradient extends dart.Object {
   }
-
   class CanvasPattern extends dart.Object {
   }
-
   class TextMetrics extends dart.Object {
   }
-
   class ImageData extends dart.Object {
   }
-
   // Exports:
   dom.JsType = JsType;
   dom.JsGlobal = JsGlobal;