Make `flutter listen` exit nicely when missing dependencies
Make `flutter listen` exit nicely when missing dependencies

Previously it died with a confusing exception.

@chinmaygarde

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