Fix type error found by dartanalyzer --strong
Fix type error found by dartanalyzer --strong

We were declaring that the function passed to setState should return a
Function. In reality, we want the function to return void (and everyone calls
it with a function that returns void).

tvolkert/flutter,Hixie/flutter,cbracken/flutter,Hixie/flutter,cbracken/flutter,cbracken/flutter,flutter/flutter,turnidge/flutter,jason-simmons/flutter,turnidge/flutter,turnidge/flutter,tvolkert/flutter,Hixie/flutter,flutter/flutter,flutter/flutter,tvolkert/flutter,Hixie/flutter,flutter/flutter,tvolkert/flutter,flutter/flutter,cbracken/flutter,jason-simmons/flutter,jason-simmons/flutter,tvolkert/flutter,cbracken/flutter,Hixie/flutter,turnidge/flutter,cbracken/flutter,turnidge/flutter,Hixie/flutter,cbracken/flutter,tvolkert/flutter,tvolkert/flutter,aghassemi/flutter,cbracken/flutter,jason-simmons/flutter,Hixie/flutter,tvolkert/flutter,flutter/flutter,aghassemi/flutter,aghassemi/flutter,flutter/flutter,cbracken/flutter,flutter/flutter,flutter/flutter,aghassemi/flutter,cbracken/flutter,flutter/flutter,tvolkert/flutter,tvolkert/flutter,Hixie/flutter,Hixie/flutter,Hixie/flutter