use js_ast instead of strings to generate JS

test/codegen/expect/dom/dom.js

 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;
       this.globalCompositeOperation = null;
       this.strokeStyle = null;
       this.fillStyle = null;
       this.shadowOffsetX = null;
       this.shadowOffsetY = null;
       this.shadowBlur = null;
       this.shadowColor = null;
     }
   }
-
   class CanvasDrawingStyles extends dart.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 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;
   dom.Overload = Overload;
   dom.overload = overload;
   dom.Document = Document;
   dom.Element = Element;
   dom.Event = Event;
   dom.InputElement = InputElement;
   dom.CanvasElement = CanvasElement;
   dom.RenderingContext = RenderingContext;
   dom.CanvasRenderingContext2D = CanvasRenderingContext2D;
   dom.CanvasDrawingStyles = CanvasDrawingStyles;
   dom.CanvasPathMethods = CanvasPathMethods;
   dom.CanvasGradient = CanvasGradient;
   dom.CanvasPattern = CanvasPattern;
   dom.TextMetrics = TextMetrics;
   dom.ImageData = ImageData;
 })(dom || (dom = {}));