use js_ast instead of strings to generate JS

test/codegen/expect/BenchmarkBase/BenchmarkBase.js

 var BenchmarkBase;
-(function (BenchmarkBase) {
+(function(BenchmarkBase) {
   'use strict';
   class Expect extends dart.Object {
     static equals(expected, actual) {
       if (!dart.equals(expected, actual)) {
         throw `Values not equal: ${expected} vs ${actual}`;
       }
     }
     static listEquals(expected, actual) {
       if (expected.length !== actual.length) {
         throw `Lists have different lengths: ${expected.length} vs ${actual.length}`;
       }
       for (let i = 0; i < actual.length; i++) {
         equals(expected.get(i), actual.get(i));
       }
     }
     fail(message) {
       throw message;
     }
   }
-
   class BenchmarkBase extends dart.Object {
     BenchmarkBase(name) {
       this.name = name;
     }
-    run() {
-    }
+    run() {}
     warmup() {
       this.run();
     }
     exercise() {
       for (let i = 0; i < 10; i++) {
         this.run();
       }
     }
-    setup() {
-    }
-    teardown() {
-    }
+    setup() {}
+    teardown() {}
     static measureFor(f, timeMinimum) {
       let time = 0;
       let iter = 0;
       let watch = new core.Stopwatch();
       watch.start();
       let elapsed = 0;
       while (elapsed < timeMinimum) {
         dart.dinvokef(f);
         elapsed = watch.elapsedMilliseconds;
         iter++;
       }
       return 1000.0 * elapsed / iter;
     }
     measure() {
       this.setup();
       measureFor((() => {
         this.warmup();
       }).bind(this), 100);
       let result = measureFor((() => {
         this.exercise();
       }).bind(this), 2000);
       this.teardown();
       return result;
     }
     report() {
       let score = this.measure();
       core.print(`${this.name}(RunTime): ${score} us.`);
     }
   }
-
   // Exports:
   BenchmarkBase.Expect = Expect;
   BenchmarkBase.BenchmarkBase = BenchmarkBase;
 })(BenchmarkBase || (BenchmarkBase = {}));