Make frontend_server depend on dart/pkg/* files (#4369)
Make frontend_server depend on dart/pkg/* files (#4369)

It speeds up local development workflow when doing changes to the front end
when ninja automatically rebuilds the frontend_server.dart.snapshot.
cdotstout/sky_engine,jamesr/flutter_engine,chinmaygarde/sky_engine,jason-simmons/sky_engine,devoncarew/sky_engine,cdotstout/sky_engine,devoncarew/sky_engine,chinmaygarde/sky_engine,aam/engine,jamesr/sky_engine,chinmaygarde/flutter_engine,tvolkert/engine,jason-simmons/flutter_engine,Hixie/sky_engine,Hixie/sky_engine,cdotstout/sky_engine,tvolkert/engine,jamesr/flutter_engine,krisgiesing/sky_engine,devoncarew/sky_engine,tvolkert/engine,aam/engine,jamesr/flutter_engine,chinmaygarde/sky_engine,flutter/engine,jason-simmons/sky_engine,jamesr/sky_engine,tvolkert/engine,aam/engine,jason-simmons/sky_engine,flutter/engine,aam/engine,mikejurka/engine,rmacnak-google/engine,cdotstout/sky_engine,chinmaygarde/sky_engine,mikejurka/engine,Hixie/sky_engine,aam/engine,Hixie/sky_engine,Hixie/sky_engine,krisgiesing/sky_engine,devoncarew/engine,chinmaygarde/flutter_engine,aam/engine,mikejurka/engine,rmacnak-google/engine,mikejurka/engine,tvolkert/engine,rmacnak-google/engine,jamesr/sky_engine,jason-simmons/flutter_engine,rmacnak-google/engine,aam/engine,chinmaygarde/flutter_engine,krisgiesing/sky_engine,rmacnak-google/engine,jason-simmons/flutter_engine,krisgiesing/sky_engine,mikejurka/engine,aam/engine,Hixie/sky_engine,jason-simmons/sky_engine,chinmaygarde/flutter_engine,jason-simmons/sky_engine,jamesr/flutter_engine,flutter/engine,jason-simmons/sky_engine,chinmaygarde/sky_engine,jamesr/flutter_engine,chinmaygarde/flutter_engine,Hixie/sky_engine,jamesr/flutter_engine,jamesr/flutter_engine,jason-simmons/flutter_engine,mikejurka/engine,flutter/engine,krisgiesing/sky_engine,devoncarew/engine,jason-simmons/flutter_engine,chinmaygarde/flutter_engine,jason-simmons/flutter_engine,cdotstout/sky_engine,jamesr/sky_engine,devoncarew/sky_engine,krisgiesing/sky_engine,jamesr/sky_engine,devoncarew/sky_engine,devoncarew/engine,devoncarew/engine,jamesr/flutter_engine,jason-simmons/flutter_engine,cdotstout/sky_engine,jamesr/flutter_engine,tvolkert/engine,rmacnak-google/engine,flutter/engine,mikejurka/engine,mikejurka/engine,jason-simmons/sky_engine,chinmaygarde/sky_engine,devoncarew/engine,Hixie/sky_engine,tvolkert/engine,jamesr/sky_engine,devoncarew/sky_engine,jason-simmons/flutter_engine,devoncarew/sky_engine,flutter/engine,chinmaygarde/sky_engine,jamesr/sky_engine,devoncarew/engine,cdotstout/sky_engine,flutter/engine,chinmaygarde/flutter_engine,mikejurka/engine,flutter/engine,devoncarew/engine,krisgiesing/sky_engine,rmacnak-google/engine