Expand Semantics dartdocs. (#6399)
Expand Semantics dartdocs. (#6399)

I based this on a conversation I had with @mpcomplete.
flutter/flutter,flutter/flutter,tvolkert/flutter,tvolkert/flutter,turnidge/flutter,tvolkert/flutter,tvolkert/flutter,tvolkert/flutter,Hixie/flutter,tvolkert/flutter,Hixie/flutter,flutter/flutter,Hixie/flutter,cbracken/flutter,Hixie/flutter,cbracken/flutter,flutter/flutter,cbracken/flutter,flutter/flutter,Hixie/flutter,Hixie/flutter,cbracken/flutter,Hixie/flutter,turnidge/flutter,Hixie/flutter,Hixie/flutter,turnidge/flutter,turnidge/flutter,flutter/flutter,tvolkert/flutter,flutter/flutter,tvolkert/flutter,flutter/flutter,tvolkert/flutter,cbracken/flutter,turnidge/flutter,Hixie/flutter,cbracken/flutter,flutter/flutter,flutter/flutter,cbracken/flutter,cbracken/flutter,cbracken/flutter,cbracken/flutter,tvolkert/flutter