Beta 11 --> Beta 12: Leading slash required in paths for "Add Alias" page (and elsewhere)

The Add Alias page that can be accessed via admin/config/search/path/add  now requires that values entered for both of the fields Existing System Path and Path Alias must start with a leading slash. Otherwise, the form does not validate.

When filling in the form manually, error messages are displayed to that effect, so it's clear what the problem is.

However, when this is being done programmatically in an automated Testing case, it's not at all obvious what went wrong.

Here's a piece of original code to create a path alias to an existing node, written for a subclass of WebTestBase:

  $edit = array();
  $edit['source'] = 'node/' . $node->id();
  $edit['alias'] = $this->randomMachineName(10);
  $this->drupalPostForm($this->addAliasPage, $edit, t('Save'));

 And here's the corrected code:

  $edit = array();
  $edit['source'] = '/node/' . $node->id();
  $edit['alias'] = '/' . $this->randomMachineName(10);
  $this->drupalPostForm($this->addAliasPage, $edit, t('Save'));

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There seems to be an overall pattern that paths relative to the site root must now start with a slash. Besides these fields for the Add Alias page, I wrote in a previous post about such a requirement in calls to some methods of class AliasManager. And there's recent mention of this in Update the DB dump to have a leading slash for the frontpage.