Really fix the flutter dartdoc link (#22266)
Really fix the flutter dartdoc link (#22266)

It turns out that #21888 didn't really fix the link, because *this* is the README used by dartdoc for flutter.
cbracken/flutter,tvolkert/flutter,flutter/flutter,tvolkert/flutter,flutter/flutter,cbracken/flutter,cbracken/flutter,cbracken/flutter,flutter/flutter,flutter/flutter,flutter/flutter,cbracken/flutter,cbracken/flutter,cbracken/flutter,Hixie/flutter,Hixie/flutter,tvolkert/flutter,tvolkert/flutter,cbracken/flutter,cbracken/flutter,tvolkert/flutter,Hixie/flutter,tvolkert/flutter,flutter/flutter,flutter/flutter,Hixie/flutter,flutter/flutter,Hixie/flutter,tvolkert/flutter,Hixie/flutter,flutter/flutter,flutter/flutter,tvolkert/flutter,Hixie/flutter,cbracken/flutter,Hixie/flutter,Hixie/flutter,tvolkert/flutter,tvolkert/flutter,Hixie/flutter