Emit macOS version information in doctor (#8772)
Emit macOS version information in doctor (#8772)

macOS counterpart to c670cd9e6458c6e2cebf9e7c67870a625289a2c9.
Hixie/flutter,flutter/flutter,Hixie/flutter,flutter/flutter,cbracken/flutter,tvolkert/flutter,flutter/flutter,flutter/flutter,cbracken/flutter,cbracken/flutter,cbracken/flutter,flutter/flutter,flutter/flutter,Hixie/flutter,Hixie/flutter,turnidge/flutter,tvolkert/flutter,Hixie/flutter,tvolkert/flutter,tvolkert/flutter,cbracken/flutter,flutter/flutter,turnidge/flutter,cbracken/flutter,turnidge/flutter,turnidge/flutter,Hixie/flutter,Hixie/flutter,tvolkert/flutter,tvolkert/flutter,cbracken/flutter,Hixie/flutter,tvolkert/flutter,cbracken/flutter,tvolkert/flutter,turnidge/flutter,tvolkert/flutter,cbracken/flutter,Hixie/flutter,flutter/flutter,tvolkert/flutter,flutter/flutter,cbracken/flutter,flutter/flutter,Hixie/flutter