code: def run(self,
              env: env_tools.PreparedEnv,
              verbose: bool,
              previous_failures: Set['Check']) -> CheckResult:
          
  
          # Skip if a dependency failed.
          if previous_failures.intersection(self.dependencies):
              print(shell_tools.highlight(
                  'Skipped ' + self.command_line_switch(),
                  shell_tools.YELLOW))
              return CheckResult(
                  self, False, 'Skipped due to dependency failing.', None)
  
          print(shell_tools.highlight(
              'Running ' + self.command_line_switch(),
              shell_tools.GREEN))
          try:
              success, message = self.perform_check(env, verbose=verbose)
              result = CheckResult(self, success, message, None)
          except Exception as ex:
              result = CheckResult(self, False, 'Unexpected error.', ex)
  
          print(shell_tools.highlight(
              'Finished ' + self.command_line_switch(),
              shell_tools.GREEN if result.success else shell_tools.RED))
          if verbose:
              print(result)
  
          return result
docs: Evaluates this check.
  
          Args:
              env: The prepared python environment to run the check in.
              verbose: When set, more progress output is produced.
              previous_failures: Checks that have already run and failed.
  
          Returns:
              A CheckResult instance.