diff -u b/core/lib/Drupal/Core/Breadcrumb/Breadcrumb.php b/core/lib/Drupal/Core/Breadcrumb/Breadcrumb.php
--- b/core/lib/Drupal/Core/Breadcrumb/Breadcrumb.php
+++ b/core/lib/Drupal/Core/Breadcrumb/Breadcrumb.php
@@ -8,6 +8,7 @@
 namespace Drupal\Core\Breadcrumb;
 
 use Drupal\Core\Cache\CacheableMetadata;
+use Drupal\Core\Link;
 
 /**
  * Used to return generated breadcrumbs, along with associated cacheability metadata.
@@ -27,7 +28,7 @@
    * @return \Drupal\Core\Link[]
    */
   public function getLinks() {
-    return $this->links ;
+    return $this->links;
   }
 
   /**
@@ -44,4 +45,18 @@
     return $this;
   }
+
+  /**
+   * Add a single link to the end of the breadcrumb.
+   *
+   * @param \Drupal\Core\Link $link
+   *   The link added to the breadcrumb.
+   *
+   * @return $this
+   */
+  public function addLink(Link $link) {
+    $this->links[] = $link;
+
+    return $this;
+  }
 
 }
diff -u b/core/lib/Drupal/Core/Breadcrumb/BreadcrumbBuilderInterface.php b/core/lib/Drupal/Core/Breadcrumb/BreadcrumbBuilderInterface.php
--- b/core/lib/Drupal/Core/Breadcrumb/BreadcrumbBuilderInterface.php
+++ b/core/lib/Drupal/Core/Breadcrumb/BreadcrumbBuilderInterface.php
@@ -32,7 +32,7 @@
    * @param \Drupal\Core\Routing\RouteMatchInterface $route_match
    *   The current route match.
    *
-   * @return mixed[]
+   * @return \Drupal\Core\Breadcrumb\Breadcrumb
    *   A breadcrumb.
    */
   public function build(RouteMatchInterface $route_match);
diff -u b/core/lib/Drupal/Core/Breadcrumb/BreadcrumbManager.php b/core/lib/Drupal/Core/Breadcrumb/BreadcrumbManager.php
--- b/core/lib/Drupal/Core/Breadcrumb/BreadcrumbManager.php
+++ b/core/lib/Drupal/Core/Breadcrumb/BreadcrumbManager.php
@@ -78,6 +78,7 @@
   public function build(RouteMatchInterface $route_match) {
     $build = [
       'links' => [],
+      'contexts' => [],
     ];
     $context = array('builder' => NULL);
     // Call the build method of registered breadcrumb builders,
@@ -91,7 +92,8 @@
       $breadcrumb = $builder->build($route_match);
 
       if ($breadcrumb instanceof Breadcrumb) {
-        // The builder returned an array of breadcrumb links.
+        // The builder returned a Breadcrumb object which has to be added to an
+        // array so it could be altered.
         $build['contexts'] = $breadcrumb->getCacheContexts();
         $build['links'] = $breadcrumb->getLinks();
         $context['builder'] = $builder;
@@ -103,8 +105,13 @@
     }
     // Allow modules to alter the breadcrumb.
     $this->moduleHandler->alter('system_breadcrumb', $build['links'], $route_match, $context);
-    // Fall back to an empty breadcrumb.
-    return $build;
+
+    // Create new Breadcrumb object.
+    $breadcrumb = new Breadcrumb();
+    $breadcrumb->setLinks($build['links']);
+    $breadcrumb->setCacheContexts($build['contexts']);
+
+    return $breadcrumb;
   }
 
   /**
diff -u b/core/modules/forum/src/Breadcrumb/ForumNodeBreadcrumbBuilder.php b/core/modules/forum/src/Breadcrumb/ForumNodeBreadcrumbBuilder.php
--- b/core/modules/forum/src/Breadcrumb/ForumNodeBreadcrumbBuilder.php
+++ b/core/modules/forum/src/Breadcrumb/ForumNodeBreadcrumbBuilder.php
@@ -7,7 +7,6 @@
 
 namespace Drupal\forum\Breadcrumb;
 
-use Drupal\Core\Breadcrumb\Breadcrumb;
 use Drupal\Core\Link;
 use Drupal\Core\Routing\RouteMatchInterface;
 
@@ -29,23 +28,22 @@
    * {@inheritdoc}
    */
   public function build(RouteMatchInterface $route_match) {
-    $breadcrumb = new Breadcrumb();
+    $breadcrumb= parent::build($route_match);
     $breadcrumb->setCacheContexts(['route']);
-    $links = parent::build($route_match);
 
     $parents = $this->forumManager->getParents($route_match->getParameter('node')->forum_tid);
     if ($parents) {
       $parents = array_reverse($parents);
       foreach ($parents as $parent) {
-        $links[] = Link::createFromRoute($parent->label(), 'forum.page',
+        $breadcrumb->addLink(Link::createFromRoute($parent->label(), 'forum.page',
           array(
             'taxonomy_term' => $parent->id(),
           )
-        );
+        ));
       }
     }
 
-    return $breadcrumb->setLinks($links);
+    return $breadcrumb;
   }
 
 }
diff -u b/core/modules/system/tests/src/Unit/Breadcrumbs/PathBasedBreadcrumbBuilderTest.php b/core/modules/system/tests/src/Unit/Breadcrumbs/PathBasedBreadcrumbBuilderTest.php
--- b/core/modules/system/tests/src/Unit/Breadcrumbs/PathBasedBreadcrumbBuilderTest.php
+++ b/core/modules/system/tests/src/Unit/Breadcrumbs/PathBasedBreadcrumbBuilderTest.php
@@ -131,7 +131,7 @@
       ->will($this->returnValue('/'));
 
     $links = $this->builder->build($this->getMock('Drupal\Core\Routing\RouteMatchInterface'));
-    $this->assertEquals(array(), $links);
+    $this->assertEquals([], $links->getLinks());
   }
 
   /**
@@ -145,7 +145,7 @@
       ->will($this->returnValue('/example'));
 
     $links = $this->builder->build($this->getMock('Drupal\Core\Routing\RouteMatchInterface'));
-    $this->assertEquals(array(0 => new Link('Home', new Url('<front>'))), $links);
+    $this->assertEquals([0 => new Link('Home', new Url('<front>'))], $links->getLinks());
   }
 
   /**
@@ -177,7 +177,7 @@
     $this->setupAccessManagerToAllow();
 
     $links = $this->builder->build($this->getMock('Drupal\Core\Routing\RouteMatchInterface'));
-    $this->assertEquals(array(0 => new Link('Home', new Url('<front>')), 1 => new Link('Example', new Url('example'))), $links);
+    $this->assertEquals([0 => new Link('Home', new Url('<front>')), 1 => new Link('Example', new Url('example'))], $links->getLinks());
   }
 
   /**
@@ -217,11 +217,11 @@
     $this->setupAccessManagerToAllow();
 
     $links = $this->builder->build($this->getMock('Drupal\Core\Routing\RouteMatchInterface'));
-    $this->assertEquals(array(
+    $this->assertEquals([
       new Link('Home', new Url('<front>')),
       new Link('Example', new Url('example')),
       new Link('Bar', new Url('example_bar')),
-    ), $links);
+    ], $links->getLinks());
   }
 
   /**
@@ -245,7 +245,7 @@
     $links = $this->builder->build($this->getMock('Drupal\Core\Routing\RouteMatchInterface'));
 
     // No path matched, though at least the frontpage is displayed.
-    $this->assertEquals(array(0 => new Link('Home', new Url('<front>'))), $links);
+    $this->assertEquals([0 => new Link('Home', new Url('<front>'))], $links->getLinks());
   }
 
   /**
@@ -286,7 +286,7 @@
     $links = $this->builder->build($this->getMock('Drupal\Core\Routing\RouteMatchInterface'));
 
     // No path matched, though at least the frontpage is displayed.
-    $this->assertEquals(array(0 => new Link('Home', new Url('<front>'))), $links);
+    $this->assertEquals([0 => new Link('Home', new Url('<front>'))], $links->getLinks());
   }
 
   /**
@@ -331,7 +331,7 @@
       ->will($this->returnValue('Admin'));
 
     $links = $this->builder->build($this->getMock('Drupal\Core\Routing\RouteMatchInterface'));
-    $this->assertEquals(array(0 => new Link('Home', new Url('<front>')), 1 => new Link('Admin', new Url('user_page'))), $links);
+    $this->assertEquals([0 => new Link('Home', new Url('<front>')), 1 => new Link('Admin', new Url('user_page'))], $links->getLinks());
   }
 
   /**
diff -u b/core/tests/Drupal/Tests/Core/Breadcrumb/BreadcrumbManagerTest.php b/core/tests/Drupal/Tests/Core/Breadcrumb/BreadcrumbManagerTest.php
--- b/core/tests/Drupal/Tests/Core/Breadcrumb/BreadcrumbManagerTest.php
+++ b/core/tests/Drupal/Tests/Core/Breadcrumb/BreadcrumbManagerTest.php
@@ -52,7 +52,7 @@
    */
   public function testBuildWithoutBuilder() {
     $result = $this->breadcrumbManager->build($this->getMock('Drupal\Core\Routing\RouteMatchInterface'));
-    $this->assertEquals(array(), $result['links']);
+    $this->assertEquals(array(), $result->getLinks());
   }
 
   /**
@@ -79,7 +79,7 @@
     $this->breadcrumbManager->addBuilder($builder, 0);
 
     $result = $this->breadcrumbManager->build($route_match);
-    $this->assertEquals($links, $result['links']);
+    $this->assertEquals($links, $result->getLinks());
   }
 
   /**
@@ -112,7 +112,7 @@
     $this->breadcrumbManager->addBuilder($builder2, 10);
 
     $result = $this->breadcrumbManager->build($route_match);
-    $this->assertEquals($links2, $result['links']);
+    $this->assertEquals($links2, $result->getLinks());
   }
 
   /**
@@ -146,7 +146,7 @@
     $this->breadcrumbManager->addBuilder($builder2, 0);
 
     $result = $this->breadcrumbManager->build($route_match);
-    $this->assertEquals($links2, $result['links']);
+    $this->assertEquals($links2, $result->getLinks());
   }
 
   /**
only in patch2:
unchanged:
--- a/core/modules/forum/src/Breadcrumb/ForumListingBreadcrumbBuilder.php
+++ b/core/modules/forum/src/Breadcrumb/ForumListingBreadcrumbBuilder.php
@@ -7,6 +7,7 @@
 
 namespace Drupal\forum\Breadcrumb;
 
+use Drupal\Core\Breadcrumb\Breadcrumb;
 use Drupal\Core\Link;
 use Drupal\Core\Routing\RouteMatchInterface;
 
@@ -34,9 +35,9 @@ public function build(RouteMatchInterface $route_match) {
     if ($parents) {
       foreach (array_reverse($parents) as $parent) {
         if ($parent->id() != $term_id) {
-          $breadcrumb[] = Link::createFromRoute($parent->label(), 'forum.page', array(
+          $breadcrumb->addLink(Link::createFromRoute($parent->label(), 'forum.page', array(
             'taxonomy_term' => $parent->id(),
-          ));
+          )));
         }
       }
     }
only in patch2:
unchanged:
--- a/core/modules/forum/tests/src/Unit/Breadcrumb/ForumListingBreadcrumbBuilderTest.php
+++ b/core/modules/forum/tests/src/Unit/Breadcrumb/ForumListingBreadcrumbBuilderTest.php
@@ -197,7 +197,7 @@ public function testBuild() {
       Link::createFromRoute('Fora_is_the_plural_of_forum', 'forum.index'),
       Link::createFromRoute('Something', 'forum.page', array('taxonomy_term' => 1)),
     );
-    $this->assertEquals($expected1, $breadcrumb_builder->build($route_match));
+    $this->assertEquals($expected1, $breadcrumb_builder->build($route_match)->getLinks());
 
     // Second test.
     $expected2 = array(
@@ -206,7 +206,7 @@ public function testBuild() {
       Link::createFromRoute('Something else', 'forum.page', array('taxonomy_term' => 2)),
       Link::createFromRoute('Something', 'forum.page', array('taxonomy_term' => 1)),
     );
-    $this->assertEquals($expected2, $breadcrumb_builder->build($route_match));
+    $this->assertEquals($expected2, $breadcrumb_builder->build($route_match)->getLinks());
   }
 
 }
only in patch2:
unchanged:
--- a/core/modules/forum/tests/src/Unit/Breadcrumb/ForumNodeBreadcrumbBuilderTest.php
+++ b/core/modules/forum/tests/src/Unit/Breadcrumb/ForumNodeBreadcrumbBuilderTest.php
@@ -9,7 +9,6 @@
 
 use Drupal\Core\Link;
 use Drupal\Tests\UnitTestCase;
-use Symfony\Cmf\Component\Routing\RouteObjectInterface;
 
 /**
  * @coversDefaultClass \Drupal\forum\Breadcrumb\ForumNodeBreadcrumbBuilder
@@ -162,6 +161,8 @@ public function testBuild() {
         array('taxonomy_vocabulary', $vocab_storage),
       )));
 
+
+
     $config_factory = $this->getConfigFactoryStub(
       array(
         'forum.settings' => array(
@@ -203,7 +204,7 @@ public function testBuild() {
       Link::createFromRoute('Forums', 'forum.index'),
       Link::createFromRoute('Something', 'forum.page', array('taxonomy_term' => 1)),
     );
-    $this->assertEquals($expected1, $breadcrumb_builder->build($route_match));
+    $this->assertEquals($expected1, $breadcrumb_builder->build($route_match)->getLinks());
 
     // Second test.
     $expected2 = array(
@@ -212,7 +213,7 @@ public function testBuild() {
       Link::createFromRoute('Something else', 'forum.page', array('taxonomy_term' => 2)),
       Link::createFromRoute('Something', 'forum.page', array('taxonomy_term' => 1)),
     );
-    $this->assertEquals($expected2, $breadcrumb_builder->build($route_match));
+    $this->assertEquals($expected2, $breadcrumb_builder->build($route_match)->getLinks());
   }
 
 }