use js_ast instead of strings to generate JS

test/codegen/expect/sunflower/sunflower.js

 var sunflower;
-(function (sunflower) {
+(function(sunflower) {
   'use strict';
   let ORANGE = "orange";
   let SEED_RADIUS = 2;
   let SCALE_FACTOR = 4;
   let TAU = math.PI * 2;
   let MAX_D = 300;
   let centerX = MAX_D / 2;
   let centerY = centerX;
   // Function querySelector: (String) → Element
-  function querySelector(selector) { return dom.document.querySelector(selector); }
-
+  function querySelector(selector) {
+    return dom.document.querySelector(selector);
+  }
   sunflower.seeds = 0;
   dart.defineLazyProperties(sunflower, {
-    get slider() { return dart.as(querySelector("#slider"), dom.InputElement) },
-    get notes() { return querySelector("#notes") },
-    get PHI() { return (math.sqrt(5) + 1) / 2 },
-    get context() { return dart.as((dart.as(querySelector("#canvas"), dom.CanvasElement)).getContext('2d'), dom.CanvasRenderingContext2D) },
+    get slider() {
+      return dart.as(querySelector("#slider"), dom.InputElement);
+    },
+    get notes() {
+      return querySelector("#notes");
+    },
+    get PHI() {
+      return (math.sqrt(5) + 1) / 2;
+    },
+    get context() {
+      return dart.as(dart.as(querySelector("#canvas"), dom.CanvasElement).getContext('2d'), dom.CanvasRenderingContext2D);
+    }
   });
-
   class Circle extends dart.Object {
     Circle(x, y, radius) {
       this.x = x;
       this.y = y;
       this.radius = radius;
     }
   }
-
   class CirclePainter extends dart.Object {
     CirclePainter() {
       this.color = ORANGE;
     }
     draw() {
       sunflower.context.beginPath();
       sunflower.context.lineWidth = 2;
       sunflower.context.fillStyle = this.color;
       sunflower.context.strokeStyle = this.color;
       sunflower.context.arc(this.x, this.y, this.radius, 0, TAU, false);
       sunflower.context.fill();
       sunflower.context.closePath();
       sunflower.context.stroke();
     }
   }
-
   class SunflowerSeed extends dart.mixin(Circle, CirclePainter) {
     SunflowerSeed(x, y, radius, color) {
-      if (color === undefined) color = null;
+      if (color === void 0)
+        color = null;
       super.Circle(x, y, radius);
-      if (color !== null) this.color = color;
+      if (color !== null)
+        this.color = color;
     }
   }
-
   // Function main: () → void
   function main() {
     sunflower.slider.addEventListener('change', (e) => draw());
     draw();
   }
-
   // Function draw: () → void
   function draw() {
     sunflower.seeds = core.int.parse(sunflower.slider.value);
     sunflower.context.clearRect(0, 0, MAX_D, MAX_D);
     for (let i = 0; i < sunflower.seeds; i++) {
       let theta = dart.notNull(i * dart.notNull(TAU)) / dart.notNull(sunflower.PHI);
       let r = math.sqrt(i) * SCALE_FACTOR;
       let x = dart.notNull(centerX) + dart.notNull(dart.notNull(r) * math.cos(theta));
       let y = dart.notNull(centerY) - dart.notNull(dart.notNull(r) * math.sin(theta));
       new SunflowerSeed(x, y, SEED_RADIUS).draw();
     }
     sunflower.notes.textContent = `${sunflower.seeds} seeds`;
   }
-
   // Exports:
   sunflower.ORANGE = ORANGE;
   sunflower.SEED_RADIUS = SEED_RADIUS;
   sunflower.SCALE_FACTOR = SCALE_FACTOR;
   sunflower.TAU = TAU;
   sunflower.MAX_D = MAX_D;
   sunflower.centerX = centerX;
   sunflower.centerY = centerY;
   sunflower.querySelector = querySelector;
   sunflower.Circle = Circle;
   sunflower.CirclePainter = CirclePainter;
   sunflower.SunflowerSeed = SunflowerSeed;
   sunflower.main = main;
   sunflower.draw = draw;
 })(sunflower || (sunflower = {}));