use js_ast instead of strings to generate JS

test/codegen/expect/html_input_c/html_input_c.js

 var html_input_c;
-(function (html_input_c) {
+(function(html_input_c) {
   'use strict';
   dart.defineLazyProperties(html_input_c, {
-    get y() { return 3 + html_input_e.w },
-    set y(x) {},
+    get y() {
+      return 3 + html_input_e.w;
+    },
+    set y() {}

[vsm, 2015/02/25 18:26:33]

Lost the param to the setter.

[Jennifer Messerly, 2015/02/25 18:33:53]

yeah ... it's kind of on purpose, because this is our dummy setter. It's our way
of indicating that this particular lazy property is writable. That said, if we
can enforce read-only for static fields at compile time, we could just always
have it be writable on the JS side.

The corresponding Dart code for this is:

    int y = 3 + w;

   });
-
 })(html_input_c || (html_input_c = {}));