Improve the asset for build functions that return null
Improve the asset for build functions that return null

Fixes #689

aghassemi/flutter,flutter/flutter,flutter/flutter,tvolkert/flutter,flutter/flutter,Hixie/flutter,tvolkert/flutter,Hixie/flutter,cbracken/flutter,tvolkert/flutter,Hixie/flutter,turnidge/flutter,flutter/flutter,Hixie/flutter,flutter/flutter,tvolkert/flutter,cbracken/flutter,turnidge/flutter,tvolkert/flutter,flutter/flutter,tvolkert/flutter,flutter/flutter,cbracken/flutter,turnidge/flutter,turnidge/flutter,flutter/flutter,cbracken/flutter,cbracken/flutter,cbracken/flutter,aghassemi/flutter,aghassemi/flutter,turnidge/flutter,Hixie/flutter,aghassemi/flutter,tvolkert/flutter,cbracken/flutter,cbracken/flutter,tvolkert/flutter,tvolkert/flutter,Hixie/flutter,cbracken/flutter,Hixie/flutter,cbracken/flutter,Hixie/flutter,Hixie/flutter,flutter/flutter,Hixie/flutter,tvolkert/flutter,flutter/flutter