upgrades core to 8.4.2

core/modules/serialization/config/install/serialization.settings.yml

@@ -2,3 +2,10 @@
 # this was usually returned from database storage. A primitive data normalizer
 # has been introduced to get the casted value instead.
 bc_primitives_as_strings: false
+# Before Drupal 8.3, timestamps were always returned as Unix timestamps, which
+# are not a universal format for interchange. Now, RFC3339 timestamps are
+# returned. New Drupal installations opt out from this by default (hence this
+# is set to false), existing installations opt in to it.
+# @see serialization_update_8301()
+# @see https://www.drupal.org/node/2768651
+bc_timestamp_normalizer_unix: false

core/modules/serialization/config/schema/serialization.schema.yml

@@ -5,3 +5,6 @@ serialization.settings:
     bc_primitives_as_strings:
       type: boolean
       label: 'Whether to retain pre Drupal 8.3 behavior of serializing all primitive items as strings.'
+    bc_timestamp_normalizer_unix:
+      type: boolean
+      label: 'Whether the pre Drupal 8.4 behavior of returning Unix timestamps instead of RFC3339 timestamps for TimestampItem fields is enabled or not.'