lib2.dart

#library('lib2');
#source('lib2_helper.dart');

String lib2String = "hello from lib2";

lib2_helper.dart

class lib2_helper {
  help() {
    print(lib2);
  }
}

main.dart

#import('lib1.dart');
#import('lib2.dart');

void main() {
  print(lib2String);
}