Check for valid field types

Hello @dpup, 

Requesting review on this b/c I'm not totally sure I want to do
it this way. wdyt?

Please review the following commits I made in branch 'nick-imports'.

16daab84ad69e2c86688dac53373cb2931668d8f (2013-09-19 17:16:24 -0400)
Check for valid field types

Check list:


R=@dpup