We have Dart code in `angular2` module that ought to be in its own
package. Examples include Dart analysis plugins, and potentially the
transformers (although transformers cannot be moved out just yet).
However, this code is Dart-only and it doesn’t make sense to use JS
directory layout for it. This commit introduces a sub-directory called
`modules_dart`. All modules in this directory are pure Dart packages
using standard pub directory layout. The code in these packages never
gets transpiled. It is directly copied to `dist` unmodified, except an
adjustment in relative paths in `pubspec.yaml` files.
yjbanov
Tobias Bosch