use js_ast instead of strings to generate JS

test/codegen/constructors.dart

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 class A {}
 
 class B {
   B();
 }
 
@@ skipping 62 matching lines @@
 class M extends L {
   M.named(int x) : super(x + 42);
 }
 
 class N extends M {
   N.named(int y) : super.named(y + 100);
 }
 
 class P extends N {
   P(int z) : super.named(z + 9000);
+  P.foo(int x) : this(x + 42);
+  P.bar() : this.foo(1);
 }