Allow suppressing message extraction warnings, providing a prefix for generated files, and format w…

pkg/intl/test/message_extraction/find_output_directory.dart

-/**
- * A shared library for finding the valu eof the --output-dir parameter
- * which all of these programs use.
- */
-library find_output_directory;
-
-const directiveName = '--output-dir=';
-
-_asString(list) => new String.fromCharCodes(list);
-
-findOutputDirectory(List<String> args) {
-  var directive = args.firstWhere(
-      (x) => x.contains(directiveName),
-      orElse: () => null);
-  if (directive == null) return '.';
-  var file = directive.codeUnits.skip(directiveName.length);
-  return _asString(file);
-}
-