anchor.diff:    stderr.writeln("Usage: dart async_compile.dart <build dir> [--silent]");
     exit(64);
   }
  +
  +/// Deletes the file at [path], ignoring any IO errors that occur.
  +///
  +/// This swallows errors to accommodate multiple compilers running concurrently.
  +/// Since they will produce the same output anyway, a failure of one is fine.
  +void _deleteFile(String path) {
  +  try {
  +    new File(path).deleteSync();
  +  } on IOException catch (ex) {
  +    // Do nothing.
  +  }
  +}
  +
  +/// Writes [contents] to [path], ignoring any IO errors that occur.
  +///
  +/// This swallows errors to accommodate multiple compilers running concurrently.
  +/// Since they will produce the same output anyway, a failure of one is fine.
  +void _writeFile(String path, String contents) {
  +  try {
  +    new File(path).writeAsStringSync(contents);
  +  } on IOException catch (ex) {
  +    // Do nothing.
  +  }
  +}
positive.code: 
  /// `true` if any file failed to compile.
  bool hadFailure = false;
  
  /// This runs the async/await compiler on all of the pub source code.
  ///
  
negative.code:     }
    }
  
    _writeFile(versionPath, COMPILER_VERSION);
  
    if (!silent) print("Compiled $numCompiled out of $numFiles files.");
  
    if (hadFailure) exit(1);
  }
  
  void _compile(String sourcePath, String source, String destPath) {
    var destDir = new Directory(p.dirname(destPath));
  