code: def file_source(self, filename):
          
          lines = []
          try:
              with self.filesystem.open(filename) as f:
                  line_statuses = dict(self.line_statuses(filename))
                  for lineno, source in enumerate(f, start=1):
                      line_status = line_statuses.get(lineno)
                      line = Line(lineno, source, line_status, None)
                      lines.append(line)
  
          except self.filesystem.FileNotFound as file_not_found:
              lines.append(
                  Line(0, '%s not found' % file_not_found.path, None, None)
              )
  
          return lines
docs: Return a list of namedtuple `Line` for each line of code found in the
          source file with the given `filename`.