diff --git a/core/modules/block/block.services.yml b/core/modules/block/block.services.yml
index fe20e4a..5674443 100644
--- a/core/modules/block/block.services.yml
+++ b/core/modules/block/block.services.yml
@@ -10,3 +10,8 @@ services:
   block.repository:
     class: Drupal\block\BlockRepository
     arguments: ['@entity.manager', '@theme.manager', '@context.handler']
+  block.main_content_context:
+    class: Drupal\block\ContextProvider\MainContentContext
+    arguments: ['@current_route_match']
+    tags:
+      - { name: 'context_provider' }
diff --git a/core/modules/block/src/ContextProvider/MainContentContext.php b/core/modules/block/src/ContextProvider/MainContentContext.php
new file mode 100644
index 0000000..547b812
--- /dev/null
+++ b/core/modules/block/src/ContextProvider/MainContentContext.php
@@ -0,0 +1,53 @@
+<?php
+
+namespace Drupal\block\ContextProvider;
+
+use Drupal\Core\Cache\CacheableMetadata;
+use Drupal\Core\Plugin\Context\Context;
+use Drupal\Core\Plugin\Context\ContextDefinition;
+use Drupal\Core\Plugin\Context\ContextProviderInterface;
+use Drupal\Core\Routing\RouteMatchInterface;
+
+/**
+ * Restricts the concept of "main content" to the block module.
+ */
+class MainContentContext implements ContextProviderInterface {
+
+  /**
+   * The route match object.
+   *
+   * @var \Drupal\Core\Routing\RouteMatchInterface
+   */
+  protected $routeMatch;
+
+  /**
+   * Constructs a new NodeRouteContext.
+   *
+   * @param \Drupal\Core\Routing\RouteMatchInterface $route_match
+   *   The route match object.
+   */
+  public function __construct(RouteMatchInterface $route_match) {
+    $this->routeMatch = $route_match;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getRuntimeContexts(array $unqualified_context_ids) {
+    $context = new Context(new ContextDefinition('boolean'), TRUE);
+    $context->addCacheableDependency((new CacheableMetadata())->setCacheContexts(['route']));
+    return ['main_content_context' => $context];
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getAvailableContexts() {
+    $contexts = [];
+    if (in_array($this->routeMatch->getRouteName(), ['block.admin_add', 'block.admin_library'])) {
+      $contexts['main_content_context'] = new Context(new ContextDefinition('boolean'));
+    }
+    return $contexts;
+  }
+
+}
diff --git a/core/modules/system/src/Plugin/Block/SystemMainBlock.php b/core/modules/block/src/Plugin/Block/SystemMainBlock.php
similarity index 78%
rename from core/modules/system/src/Plugin/Block/SystemMainBlock.php
rename to core/modules/block/src/Plugin/Block/SystemMainBlock.php
index beba38d..6316cd8 100644
--- a/core/modules/system/src/Plugin/Block/SystemMainBlock.php
+++ b/core/modules/block/src/Plugin/Block/SystemMainBlock.php
@@ -1,6 +1,6 @@
 <?php
 
-namespace Drupal\system\Plugin\Block;
+namespace Drupal\block\Plugin\Block;
 
 use Drupal\Core\Block\BlockBase;
 use Drupal\Core\Block\MainContentBlockPluginInterface;
@@ -10,7 +10,10 @@
  *
  * @Block(
  *   id = "system_main_block",
- *   admin_label = @Translation("Main page content")
+ *   admin_label = @Translation("Main page content"),
+ *   context = {
+ *     "main_content_context" = @ContextDefinition("boolean")
+ *   }
  * )
  */
 class SystemMainBlock extends BlockBase implements MainContentBlockPluginInterface {
diff --git a/core/modules/simpletest/src/BlockCreationTrait.php b/core/modules/simpletest/src/BlockCreationTrait.php
index 4b36f78..ed66669 100644
--- a/core/modules/simpletest/src/BlockCreationTrait.php
+++ b/core/modules/simpletest/src/BlockCreationTrait.php
@@ -49,6 +49,12 @@ protected function placeBlock($plugin_id, array $settings = array()) {
       'visibility' => array(),
       'weight' => 0,
     );
+
+    // Enable the main content block to be mapped correctly.
+    if ($plugin_id === 'system_main_block') {
+      $settings['context_mapping']['main_content_context'] = '@block.main_content_context:main_content_context';
+    }
+
     $values = [];
     foreach (array('region', 'id', 'theme', 'plugin', 'weight', 'visibility') as $key) {
       $values[$key] = $settings[$key];