[nnbd] build platform dill in agnostic mode (#19167)
[nnbd] build platform dill in agnostic mode (#19167)

In agnostic mode, both sound and unsound modes are supported. 
chinmaygarde/sky_engine,jamesr/flutter_engine,jamesr/flutter_engine,chinmaygarde/sky_engine,jason-simmons/sky_engine,jamesr/flutter_engine,devoncarew/engine,jason-simmons/sky_engine,flutter/engine,tvolkert/engine,jason-simmons/sky_engine,devoncarew/engine,devoncarew/sky_engine,tvolkert/engine,tvolkert/engine,chinmaygarde/flutter_engine,tvolkert/engine,aam/engine,chinmaygarde/sky_engine,devoncarew/sky_engine,aam/engine,chinmaygarde/flutter_engine,devoncarew/sky_engine,aam/engine,jason-simmons/flutter_engine,devoncarew/engine,aam/engine,jason-simmons/sky_engine,devoncarew/engine,rmacnak-google/engine,aam/engine,chinmaygarde/flutter_engine,rmacnak-google/engine,chinmaygarde/sky_engine,devoncarew/sky_engine,devoncarew/sky_engine,flutter/engine,jamesr/sky_engine,rmacnak-google/engine,chinmaygarde/flutter_engine,jason-simmons/flutter_engine,jamesr/flutter_engine,jason-simmons/flutter_engine,flutter/engine,aam/engine,rmacnak-google/engine,rmacnak-google/engine,devoncarew/engine,jason-simmons/sky_engine,jamesr/sky_engine,flutter/engine,jamesr/sky_engine,chinmaygarde/sky_engine,Hixie/sky_engine,jamesr/flutter_engine,tvolkert/engine,jamesr/flutter_engine,Hixie/sky_engine,tvolkert/engine,chinmaygarde/flutter_engine,devoncarew/sky_engine,jason-simmons/flutter_engine,Hixie/sky_engine,Hixie/sky_engine,chinmaygarde/sky_engine,jason-simmons/sky_engine,chinmaygarde/flutter_engine,flutter/engine,jason-simmons/flutter_engine,flutter/engine,jason-simmons/flutter_engine,Hixie/sky_engine,jamesr/flutter_engine,chinmaygarde/flutter_engine,Hixie/sky_engine,jason-simmons/flutter_engine,rmacnak-google/engine,rmacnak-google/engine,jamesr/sky_engine,devoncarew/engine,chinmaygarde/sky_engine,flutter/engine,jason-simmons/flutter_engine,jamesr/sky_engine,jason-simmons/sky_engine,aam/engine,jamesr/sky_engine,devoncarew/sky_engine,Hixie/sky_engine,jamesr/flutter_engine,devoncarew/engine,aam/engine,jamesr/flutter_engine,Hixie/sky_engine,tvolkert/engine,flutter/engine,jamesr/sky_engine