I'm sorry you're running into this issue. Please see the attached updated 
file and it is opening fine in Google Web Designer now.

Here are the instruction to fix the file :
Open the author file in a text edit and search to see there's any id with "
head" in it (id="head"). If there is, please rename head to head2, save it 
and the file can be open again in Google Web Designer.

Thanks
Sukhbir
(Google Web Designer Team)