use js_ast instead of strings to generate JS

test/codegen/expect/html_input_d/html_input_d.js

 var html_input_d;
-(function (html_input_d) {
+(function(html_input_d) {
   'use strict';
   // Function fib: (int) → int
-  function fib(n) { return _fib(n, new core.Map()); }
-
+  function fib(n) {
+    return _fib(n, new core.Map());
+  }
   // Function _fib: (int, Map<int, int>) → int
   function _fib(n, seen) {
-    if (dart.notNull(n === 0) || dart.notNull(n === 1)) return 1;
-    if (seen.get(n) !== null) return seen.get(n);
+    if (dart.notNull(n === 0) || dart.notNull(n === 1))
+      return 1;
+    if (seen.get(n) !== null)
+      return seen.get(n);
     seen.set(n, _fib(n - 1, seen) + _fib(n - 2, seen));
     return seen.get(n);
   }
-
   // Exports:
   html_input_d.fib = fib;
 })(html_input_d || (html_input_d = {}));