WIP Flexbox Layout Manager for Sky framework.
WIP Flexbox Layout Manager for Sky framework.

This only handles really basic cases and doesn't understand
justification, wrapping, and so on. Submitting for code
review so I can get some early feedback before I invest
more time into making it correct.

R=ianh@google.com, abarth

Review URL: https://codereview.chromium.org/1151293002

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