add the missing handler.php file

lib/Drupal/views/Plugins/views/Handler.php

+<?php
+/**
+ * @file
+ * Definition of Drupal\views\Plugins\views\Handler
+ */
+
+namespace Drupal\views\Plugins\views;
+
+use Drupal\views\Plugins\views\Plugin;
+use Drupal\views\View;
+
+class Handler extends Plugin {
+  /**
+   * init the handler with necessary data.
+   * @param $view
+   *   The $view object this handler is attached to.
+   * @param $options
+   *   The item from the database; the actual contents of this will vary
+   *   based upon the type of handler.
+   */
+  function init(&$view, &$options) {
+    $this->view = &$view;
+    $display_id = $this->view->current_display;
+    // Check to see if this handler type is defaulted. Note that
+    // we have to do a lookup because the type is singular but the
+    // option is stored as the plural.
+
+    // If the 'moved to' keyword moved our handler, let's fix that now.
+    if (isset($this->actual_table)) {
+      $options['table'] = $this->actual_table;
+    }
+
+    if (isset($this->actual_field)) {
+      $options['field'] = $this->actual_field;
+    }
+
+    $types = View::views_object_types();
+    $plural = $this->handler_type;
+    if (isset($types[$this->handler_type]['plural'])) {
+      $plural = $types[$this->handler_type]['plural'];
+    }
+    if ($this->view->display_handler->is_defaulted($plural)) {
+      $display_id = 'default';
+    }
+
+    $this->localization_keys = array(
+      $display_id,
+      $this->handler_type,
+      $options['table'],
+      $options['id']
+    );
+
+    $this->unpack_options($this->options, $options);
+
+    // This exist on most handlers, but not all. So they are still optional.
+    if (isset($options['table'])) {
+      $this->table = $options['table'];
+    }
+
+    if (isset($this->definition['real field'])) {
+      $this->real_field = $this->definition['real field'];
+    }
+
+    if (isset($this->definition['field'])) {
+      $this->real_field = $this->definition['field'];
+    }
+
+    if (isset($options['field'])) {
+      $this->field = $options['field'];
+      if (!isset($this->real_field)) {
+        $this->real_field = $options['field'];
+      }
+    }
+
+    $this->query = &$view->query;
+  }
+}