Fix packages links after web_ui compile

[kothar]

I've further extended the buildpack to include fixing all packages symlinks after build.dart is executed.

The web/out folder (at least) contains several links to packages after the web_ui compiler runs, and these need fixing up before the slug will work on Heroku.

I also have set the path to include the dart bin directory before build.dart is executed, since I run dart2js from my build.dart.

In the case where no build.dart is present, these changes should not affect the buildpack's behaviour, except to fix any extra links to packages if they are not at web/packages.

[kothar]

Squashing commits