Insertion points with select

Check it out, @shailen and @Sfshaza.

Staged at:
https://github.com/dart-lang/polymer-dart-snippets/tree/insertion_points_with_select/web/insertion_points/creating_insertion_points_using_the_select_attribute#creating-insertion-points-using-the-select-attribute
I like sticking 2 blank lines above section heads. It's not a must though... I can take it out if it looks wrong to you.