fix builder issue with packages folder changes

Fix an issue where newly created dart projects would show up with analysis errors. The issue is that the dart analyzer doesn't handle refreshing itself when content changes in the packages/ folder. The dart editor works around this issue; we will as well :) There's an open issue to push the fix into the analysis engine itself.

fixes #2602, @dinhviethoa. Also, some small template tweaks.
Ah, packages changes happen very infrequently. Just after a pub get or pub update. Maybe once a week in a project's lifecycle -