[roll] Update 3p packages

_discoveryapis_commons      0.1.5 --> 0.1.6     
            dart_style     1.0.10 --> 1.0.12    
            googleapis     0.50.0 --> 0.50.2    

Change-Id: I507296240a7ead3bdd791d1a4c2aa32a8ba02a21