Migrate: add an Iterator process plugin

It's doxygen'd,

1. +++ b/core/modules/migrate/lib/Drupal/migrate/Plugin/migrate/process/Iterator.php
   @@ -0,0 +1,56 @@
   +  /**
   +   * @param $key
   ...
   +  protected function transformKey($key, MigrateExecutable $migrate_executable, Row $row) {
   

   So nice, all the parameters are doxygen'd but what is the function actually doing?

2. +++ b/core/modules/migrate/lib/Drupal/migrate/Plugin/migrate/process/Iterator.php
   --- /dev/null
   +++ b/core/modules/migrate/tests/Drupal/migrate/Tests/process/IteratorTest.php
   
   +++ b/core/modules/migrate/tests/Drupal/migrate/Tests/process/IteratorTest.php
   @@ -0,0 +1,86 @@
   + * @file
   + * Contains
   + */
   +
   ...
   +/**
   + * @group migrate
   + * @group Drupal
   + */
   +class IteratorTest extends MigrateTestCase {
   

   It would be great to at least tell people which class is tested here.

Here are some more points and suggestions.

1. +++ b/core/modules/migrate/lib/Drupal/migrate/Plugin/migrate/process/Iterator.php
   @@ -0,0 +1,58 @@
   + */
   +
   +
   +namespace Drupal\migrate\Plugin\migrate\process;
   

   Extra space.

2. +++ b/core/modules/migrate/lib/Drupal/migrate/Plugin/migrate/process/Iterator.php
   @@ -0,0 +1,58 @@
   +   * Run the process pipeline for the current key.
   

   Runs

3. +++ b/core/modules/migrate/lib/Drupal/migrate/Plugin/migrate/process/Iterator.php
   @@ -0,0 +1,58 @@
   +   * @param $key
   

   Typehint missing.

4. +++ b/core/modules/migrate/lib/Drupal/migrate/Plugin/migrate/process/Iterator.php
   @@ -0,0 +1,58 @@
   +   * @param MigrateExecutable $migrate_executable
   ...
   +   * @param Row $row
   

   Fully namespaced class is required for typehint.

5. +++ b/core/modules/migrate/lib/Drupal/migrate/Plugin/migrate/process/Iterator.php
   @@ -0,0 +1,58 @@
   +   * @return mixed
   

   I think mix is fine but what about int|string|whatever and then in desc explain all the cases. Just a suggestion.

6. +++ b/core/modules/migrate/tests/Drupal/migrate/Tests/process/IteratorTest.php
   @@ -0,0 +1,88 @@
   + * Contains
   

   Incomplete.

7. +++ b/core/modules/migrate/tests/Drupal/migrate/Tests/process/IteratorTest.php
   @@ -0,0 +1,88 @@
   + * Test the iterator process plugin.
   ...
   +   * Test the iterator process plugin.
   

   Tests

8. +++ b/core/modules/migrate/tests/Drupal/migrate/Tests/process/IteratorTest.php
   @@ -0,0 +1,88 @@
   +   * @var \Drupal\migrate\Plugin\migrate\process\Iterator
   

   Desc missing.

9. +++ b/core/modules/migrate/tests/Drupal/migrate/Tests/process/IteratorTest.php
   @@ -0,0 +1,88 @@
   +  protected $migrationConfiguration = array(
   

   @var doc block missing.

Point 1.2 would be still nice.

Thank you

Ok, chx spent a great deal of time with me walking me through this so I could understand it. Well, sort of. :) Made some docs improvements along the way, along with a @see to https://drupal.org/node/2129651 where this is all explained in a lot more details.

Committed and pushed to 8.x. Thanks!