anchor.diff:    if (result != 0) {
       throw ProcessException(exe, args, '', result);
     }
  +  if (output != null && !File.fromUri(output).existsSync()) {
  +    throw FileSystemException(
  +      'Command succeded, but the output file is missing',
  +      output.toFilePath(),
  +    );
  +  }
   }
   
   String _toUpperUnderscore(String string) {
  
positive.code:   }
  
    // Build dart_api_dl.o.
    await _run(clang, [
      '-DDART_SHARED_LIB',
      '-DNDEBUG',
      '-fno-exceptions',
      if (os != 'windows') '-fPIC',
      '-O3',
      '-target',
      target,
      '-I',
      sdkIncDir.toFilePath(),
      '-I',
      outDir.resolve('include/').toFilePath(),
      '-c',
      sdkIncDir.resolve('dart_api_dl.c').toFilePath(),
      '-o',
      outDir.resolve('dart_api_dl.o').toFilePath()
    ]);
  
    // Build finalizers.o.
    await _run(clang, [
      '-DDART_SHARED_LIB',
      '-DNDEBUG',
      '-fno-exceptions',
      if (os != 'windows') '-fPIC',
      '-O3',
      '-target',
      target,
      '-I',
      sdkIncDir.toFilePath(),
      '-I',
      outDir.resolve('include/').toFilePath(),
      '-c',
      srcDir.resolve('finalizers.c').toFilePath(),
      '-o',
      outDir.resolve('finalizers.o').toFilePath()
    ]);
  
    // Link wasmer, dart_api_dl, and finalizers to create the output library.
    await _run(clang, [
      '-shared',
      if (args['sysroot'] != null) '--sysroot=${args['sysroot']}',
      if (os != 'windows') '-fPIC',
      if (os == 'windows') ...[
        '-lws2_32',
        '-ladvapi32',
        '-lbcrypt',
        '-luserenv',
        '-z',
        '/DEF:${srcDir.resolve('module.g.def').toFilePath()}',
        '-z',
        '/NODEFAULTLIB:MSVCRT',
      ],
      '-lm',
      '-target',
      target,
      outDir.resolve('dart_api_dl.o').toFilePath(),
      outDir.resolve('finalizers.o').toFilePath(),
      outDir.resolve('$target/release/${_getWasmerLib(os)}').toFilePath(),
      '-o',
      outLib
    ]);
  }
  
negative.code:       ? Uri.directory(args['out-dir'] as String)
        : _getOutDir(Directory.current.uri);
    final os = _getOsFromTarget(target);
    final outLib = outDir.resolve(_getOutLib(os));
    final outWasmer = outDir.resolve('$target/release/${_getWasmerLib(os)}');
    final outDartApi = outDir.resolve('dart_api_dl.o');
    final outFinalizers = outDir.resolve('finalizers.o');
  
    print('Dart SDK directory: ${sdkDir.toFilePath()}');
    print('Dart SDK include directory: ${sdkIncDir.toFilePath()}');
  