diff --cc core/modules/breakpoint/lib/Drupal/breakpoint/Entity/Breakpoint.php
index 5fa0260,36ddd1e..0000000
--- a/core/modules/breakpoint/lib/Drupal/breakpoint/Entity/Breakpoint.php
+++ b/core/modules/breakpoint/lib/Drupal/breakpoint/Entity/Breakpoint.php
@@@ -103,21 -104,9 +104,21 @@@ class Breakpoint extends ConfigEntityBa
     *
     * @var multipliers
     */
-   public $multipliers = array();
+   protected $multipliers = array();
  
    /**
 +   * {@inheritdoc}
 +   */
 +  public function id() {
 +    // If no ID is specified, build one from the properties that uniquely define
 +    // this breakpoint.
 +    if (!isset($this->id)) {
 +      $this->id = $this->sourceType . '.' . $this->source . '.' . $this->name;
 +    }
 +    return $this->id;
 +  }
 +
 +  /**
     * Overrides Drupal\config\ConfigEntityBase::save().
     */
    public function save() {
diff --cc core/modules/breakpoint/lib/Drupal/breakpoint/Tests/BreakpointAPITest.php
index e132280,5135797..0000000
--- a/core/modules/breakpoint/lib/Drupal/breakpoint/Tests/BreakpointAPITest.php
+++ b/core/modules/breakpoint/lib/Drupal/breakpoint/Tests/BreakpointAPITest.php
@@@ -6,7 -6,7 +6,7 @@@
  
  namespace Drupal\breakpoint\Tests;
  
--use Drupal\breakpoint\Tests\BreakpointsTestBase;
++use Drupal\breakpoint\Tests\BreakpointTestBase;
  use Drupal\breakpoint\Entity\Breakpoint;
  use Drupal\breakpoint\InvalidBreakpointNameException;
  use Drupal\breakpoint\InvalidBreakpointSourceException;
@@@ -46,9 -46,9 +46,9 @@@ class BreakpointAPITest extends Breakpo
      $this->assertTrue($exception, 'breakpoint_config_name: An exception is thrown when an invalid sourceType is entered.');
  
      // Try an invalid source.
 -    $breakpoint->enforceIsNew();
 +    $breakpoint = $breakpoint->createDuplicate();
-     $breakpoint->sourceType = Breakpoint::SOURCE_TYPE_USER_DEFINED;
-     $breakpoint->source = 'custom*_module source';
+     $breakpoint->setSourceType(Breakpoint::SOURCE_TYPE_USER_DEFINED)
+       ->setSource('custom*_module source');
  
      $exception = FALSE;
      try {
@@@ -60,9 -60,9 +60,9 @@@
      $this->assertTrue($exception, 'breakpoint_config_name: An exception is thrown when an invalid source is entered.');
  
      // Try an invalid name (make sure there is at least once capital letter).
 -    $breakpoint->enforceIsNew();
 +    $breakpoint = $breakpoint->createDuplicate();
-     $breakpoint->source = 'custom_module';
-     $breakpoint->name = drupal_ucfirst($this->randomName());
+     $breakpoint->setSource('custom_module')
+       ->setName(drupal_ucfirst($this->randomName()));
  
      $exception = FALSE;
      try {
@@@ -74,9 -74,9 +74,9 @@@
      $this->assertTrue($exception, 'breakpoint_config_name: An exception is thrown when an invalid name is entered.');
  
      // Try a valid breakpoint.
 -    $breakpoint->enforceIsNew();
 +    $breakpoint = $breakpoint->createDuplicate();
-     $breakpoint->name = drupal_strtolower($this->randomName());
-     $breakpoint->mediaQuery = 'all';
+     $breakpoint->setName(drupal_strtolower($this->randomName()))
+       ->setMediaQuery('all');
  
      $exception = FALSE;
      try {