Fix app bar padding without leading widget
Fix app bar padding without leading widget

If there's no leading widget, we just want 8.0 pixels of padding instead of all
the padding to get over to the 72.0 pixel key line.

Fixes #2912

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