symfony: Regression in Symfony Yaml Parser

Q	A
Bug report?	yes
Feature request?	no
BC Break report?	no
RFC?	no
Symfony version	3.3.14, 3.4.1 and 4.0.1

Hi,

Since the latest 3.3, I experiment a regression in Symfony Yaml, here’s a reproducer:

<?php
require __DIR__.'/vendor/autoload.php';

$yaml = <<<EOYAML
root:
  - key1: "bim"
    # A comment
    key2: "boum"

EOYAML;

$parser = new Symfony\Component\Yaml\Parser();
dump($parser->parse($yaml));

expected outpout:

array:1 [
  "root" => array:1 [
    0 => array:2 [
      "key1" => "bim"
      "key2" => "boum"
    ]
  ]
]

actual output:

PHP Fatal error:  Uncaught Symfony\Component\Yaml\Exception\ParseException: Unable to parse at line 4 (near "  key2: "boum""). in /Users/neutron/Documents/workspace/symfony/src/Symfony/Component/Yaml/Parser.php:426
Stack trace:
#0 /Users/neutron/Documents/workspace/symfony/src/Symfony/Component/Yaml/Parser.php(465): Symfony\Component\Yaml\Parser->doParse('- key1: "bim"\n ...', 0)
#1 /Users/neutron/Documents/workspace/symfony/src/Symfony/Component/Yaml/Parser.php(315): Symfony\Component\Yaml\Parser->parseBlock(1, '- key1: "bim"\n ...', 0)
#2 /Users/neutron/Documents/workspace/symfony/src/Symfony/Component/Yaml/Parser.php(95): Symfony\Component\Yaml\Parser->doParse('root:\n  - key1:...', 0)
#3 /Users/neutron/Documents/workspace/symfony/test.php(15): Symfony\Component\Yaml\Parser->parse('root:\n  - key1:...')
#4 {main}
  thrown in /Users/neutron/Documents/workspace/symfony/src/Symfony/Component/Yaml/Parser.php on line 426

I’ve tested against all latest versions, only version 3.3.14, 3.4.1 and 4.0.1 are affected