Improve dartdoc for GlobalKey
Improve dartdoc for GlobalKey

Fixes #2708

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