fix back button in stocks app by calling super.didMount()
fix back button in stocks app by calling super.didMount()

TBR=abarth

Review URL: https://codereview.chromium.org/1189113004.

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