Merge branch '5.1' into 5.2

* 5.1:
  Use ::class keyword when possible

Author: fabpot

Tests/Exception/FlattenExceptionTest.php

@@ -290,7 +290,7 @@ function () {},
 
         $args = $array[$i++];
         $this->assertSame('object', $args[0]);
-        $this->assertTrue('Closure' === $args[1] || is_subclass_of($args[1], '\Closure'), 'Expect object class name to be Closure or a subclass of Closure.');
+        $this->assertTrue('Closure' === $args[1] || is_subclass_of($args[1], \Closure::class), 'Expect object class name to be Closure or a subclass of Closure.');
 
         $this->assertSame(['array', [['integer', 1], ['integer', 2]]], $array[$i++]);
         $this->assertSame(['array', ['foo' => ['integer', 123]]], $array[$i++]);