refactor(dart/transform): Use targeted transformers

TL;DR: Modify pubspec.yaml files to use the recommended "targeted"
transformers. The unified "simple" angular2 transformer still works as
always, but we want to encourage use of the targeted transformers
whereever possible.

See [the wiki](https://github.com/angular/angular/wiki/Advanced-Transformer-Configuration)
for details about targeted transformers.

See #1872

modules_dart/payload/hello_world/pubspec.yaml

@@ -9,11 +9,10 @@ dependency_overrides:
   angular2:
     path: ../../../dist/dart/angular2
 transformers:
-- angular2:
+- angular2/transform/codegen:
     platform_directives: 'package:angular2/src/common/directives.dart#CORE_DIRECTIVES'
-    entry_points:
-        - web/index.dart
-
+- angular2/transform/reflection_remover:
+    $include: [web/index.dart]
 - $dart2js:
     minify: true
     commandLineOptions: