Empty files emit warning

[dickermoshe]

Empty files result in this warning

Minified Scripts
    ~/lib/src/services/database/database.dart (no sourcemap ref)
      - warning: could not determine a source map reference (Could not auto-detect referenced sourcemap for ~/lib/src/services/database/database.dart)
    ~/lib/src/startup.dart (no sourcemap ref)
      - warning: could not determine a source map reference (Could not auto-detect referenced sourcemap for ~/lib/src/startup.dart)

A warning should only be show if the file contains code.