Merge branch '3.1'

* 3.1:
  [travis] Test on PHP 7.1
  simplified code
  Remove unused variable
  3.0 Upgrade Guide: Added Table of Contents
  [HttpFoundation] add missing upgrade entry
  [FrameworkBundle] Convert null prefix to an empty string in translation:update command
  Improved the deprecation messages for service aliases
  [Serializer] minor: Remove an extra space in a YamlFileLoader error
  [HttpKernel] Clean ArgumentMetadataFactory::getType()
  [PropertyInfo] Fix edge cases in ReflectionExtractor

Author: nicolas-grekas

Command/TranslationUpdateCommand.php

@@ -132,8 +132,9 @@ protected function execute(InputInterface $input, OutputInterface $output)
         // load any messages from templates
         $extractedCatalogue = new MessageCatalogue($input->getArgument('locale'));
         $io->comment('Parsing templates...');
+        $prefix = $input->getOption('prefix');
         $extractor = $this->getContainer()->get('translation.extractor');
-        $extractor->setPrefix($input->getOption('prefix'));
+        $extractor->setPrefix(null === $prefix ? '' : $prefix);
         foreach ($transPaths as $path) {
             $path .= 'views';
             if (is_dir($path)) {