Update to Drupal 8.0.0-rc3. For more information, see https://www.drupal.org/node/2608078

core/lib/Drupal/Component/Render/FormattableMarkup.php

@@ -2,7 +2,7 @@
 
 /**
  * @file
- * Contains Drupal\Component\Render\FormattableMarkup.
+ * Contains \Drupal\Component\Render\FormattableMarkup.
  */
 
 namespace Drupal\Component\Render;
@@ -65,7 +65,7 @@ use Drupal\Component\Utility\UrlHelper;
  * @see \Drupal\Core\StringTranslation\PluralTranslatableMarkup
  * @see \Drupal\Component\Render\FormattableMarkup::placeholderFormat()
  */
-class FormattableMarkup implements MarkupInterface {
+class FormattableMarkup implements MarkupInterface, \Countable {
 
   /**
    * The arguments to replace placeholders with.
@@ -223,7 +223,6 @@ class FormattableMarkup implements MarkupInterface {
           break;
 
         case '%':
-        default:
           // Similarly to @, escape non-safe values. Also, add wrapping markup
           // in order to render as a placeholder. Not for use within attributes,
           // per the warning above about
@@ -231,6 +230,16 @@ class FormattableMarkup implements MarkupInterface {
           // due to the wrapping markup.
           $args[$key] = '<em class="placeholder">' . static::placeholderEscape($value) . '</em>';
           break;
+
+        default:
+          // We do not trigger an error for placeholder that start with an
+          // alphabetic character.
+          if (!ctype_alpha($key[0])) {
+            // We trigger an error as we may want to introduce new placeholders
+            // in the future without breaking backward compatibility.
+            trigger_error('Invalid placeholder: ' . $key, E_USER_ERROR);
+          }
+          break;
       }
     }
 

core/lib/Drupal/Component/Render/HtmlEscapedText.php

@@ -18,7 +18,7 @@ use Drupal\Component\Utility\Unicode;
  *
  * @ingroup sanitization
  */
-class HtmlEscapedText implements MarkupInterface {
+class HtmlEscapedText implements MarkupInterface, \Countable {
 
   /**
    * The string to escape.