diff --git a/core/core.libraries.yml b/core/core.libraries.yml
index 825ed77..1d3a011 100644
--- a/core/core.libraries.yml
+++ b/core/core.libraries.yml
@@ -8,7 +8,7 @@ backbone:
     url: https://github.com/jashkenas/backbone/blob/1.1.2/LICENSE
     gpl-compatible: true
   js:
-    assets/vendor/backbone/backbone-min.js: { weight: -19, minified: true }
+    assets/vendor/backbone/backbone-min.js: { minified: true }
   dependencies:
     - core/underscore
 
@@ -20,7 +20,7 @@ classList:
     url: https://github.com/eligrey/classList.js/blob/2014-12-13/LICENSE.md
     gpl-compatible: true
   js:
-    assets/vendor/classList/classList.min.js: { weight: -21, browsers: { IE: 'lte IE 9', '!IE': false }, minified: true }
+    assets/vendor/classList/classList.min.js: { browsers: { IE: 'lte IE 9', '!IE': false }, minified: true }
 
 ckeditor:
   remote: https://github.com/ckeditor/ckeditor-dev
@@ -40,12 +40,12 @@ domready:
     url: https://github.com/ded/domready/blob/v1.0.7/LICENSE
     gpl-compatible: true
   js:
-    assets/vendor/domready/ready.min.js: { weight: -21, minified: true }
+    assets/vendor/domready/ready.min.js: { minified: true }
 
 drupal:
   version: VERSION
   js:
-    misc/drupal.js: { weight: -18 }
+    misc/drupal.js: {}
   dependencies:
     - core/domready
     - core/drupalSettings
@@ -102,7 +102,7 @@ drupal.announce:
 drupal.autocomplete:
   version: VERSION
   js:
-    misc/autocomplete.js: { weight: -1 }
+    misc/autocomplete.js: {}
   dependencies:
     - core/jquery
     - core/drupal
@@ -246,7 +246,7 @@ drupal.tabbingmanager:
 drupal.tabledrag:
   version: VERSION
   js:
-    misc/tabledrag.js: { weight: -1 }
+    misc/tabledrag.js: {}
   dependencies:
     - core/jquery
     - core/modernizr
@@ -297,7 +297,7 @@ drupal.vertical-tabs:
   version: VERSION
   js:
     # Load before core/drupal.collapse.
-    misc/vertical-tabs.js: { weight: -1 }
+    misc/vertical-tabs.js: {}
   css:
     component:
       misc/vertical-tabs.css: {}
@@ -318,7 +318,7 @@ html5shiv:
     url: http://www.gnu.org/licenses/gpl-2.0.html
     gpl-compatible: true
   js:
-    assets/vendor/html5shiv/html5shiv.min.js: { every_page: true, weight: -22, browsers: { IE: 'lte IE 8', '!IE': false }, minified: true }
+    assets/vendor/html5shiv/html5shiv.min.js: { every_page: true, browsers: { IE: 'lte IE 8', '!IE': false }, minified: true }
 
 jquery:
   remote: https://github.com/jquery/jquery
@@ -328,7 +328,7 @@ jquery:
     url: https://github.com/jquery/jquery/blob/2.1.3/MIT-LICENSE.txt
     gpl-compatible: true
   js:
-    assets/vendor/jquery/jquery.min.js: { minified: true, weight: -20 }
+    assets/vendor/jquery/jquery.min.js: { minified: true }
 
 jquery.cookie:
   remote: https://github.com/carhartl/jquery-cookie
@@ -398,7 +398,7 @@ jquery.once:
     url: https://github.com/RobLoach/jquery-once/blob/1.2.3/LICENSE
     gpl-compatible: true
   js:
-    assets/vendor/jquery-once/jquery.once.js: { weight: -19 }
+    assets/vendor/jquery-once/jquery.once.js: {}
   dependencies:
     - core/jquery
 
@@ -410,7 +410,7 @@ jquery.ui:
     url: https://github.com/jquery/jquery-ui/blob/1.11.2/MIT-LICENSE.txt
     gpl-compatible: true
   js:
-    assets/vendor/jquery.ui/ui/core-min.js: { weight: -11, minified: true }
+    assets/vendor/jquery.ui/ui/core-min.js: { minified: true }
   css:
     component:
       assets/vendor/jquery.ui/themes/base/core.css: {}
@@ -510,7 +510,7 @@ jquery.ui.effects.core:
   version: *jquery_ui_version
   license: *jquery_ui_license
   js:
-    assets/vendor/jquery.ui/ui/effect-min.js: { weight: -9, minified: true }
+    assets/vendor/jquery.ui/ui/effect-min.js: { minified: true }
 
 jquery.ui.effects.blind:
   version: *jquery_ui_version
@@ -787,7 +787,7 @@ jquery.ui.widget:
   version: *jquery_ui_version
   license: *jquery_ui_license
   js:
-    assets/vendor/jquery.ui/ui/widget-min.js: { weight: -10, minified: true }
+    assets/vendor/jquery.ui/ui/widget-min.js: { minified: true }
   dependencies:
     - core/jquery.ui
 
@@ -819,7 +819,7 @@ modernizr:
     gpl-compatible: true
   version: v2.8.3
   js:
-    assets/vendor/modernizr/modernizr.min.js: { every_page: true, preprocess: 0, weight: -21, minified: true }
+    assets/vendor/modernizr/modernizr.min.js: { every_page: true, preprocess: 0 }
 
 normalize:
   remote: https://github.com/necolas/normalize.css
@@ -830,7 +830,7 @@ normalize:
     gpl-compatible: true
   css:
     base:
-      assets/vendor/normalize-css/normalize.css: { every_page: true, weight: -20 }
+      assets/vendor/normalize-css/normalize.css: { every_page: true }
 
 picturefill:
   remote: https://github.com/scottjehl/picturefill
@@ -842,7 +842,7 @@ picturefill:
     url: https://github.com/scottjehl/picturefill/blob/master/LICENSE
     gpl-compatible: true
   js:
-    assets/vendor/picturefill/picturefill.js: { weight: -10 }
+    assets/vendor/picturefill/picturefill.js: {}
   dependencies:
     - core/matchmedia
 
@@ -854,4 +854,4 @@ underscore:
     url: https://github.com/jashkenas/underscore/blob/1.7.0/LICENSE
     gpl-compatible: true
   js:
-    assets/vendor/underscore/underscore-min.js: { weight: -20, minified: true }
+    assets/vendor/underscore/underscore-min.js: { minified: true }
diff --git a/core/modules/contextual/contextual.libraries.yml b/core/modules/contextual/contextual.libraries.yml
index 16961a6..425cec7 100644
--- a/core/modules/contextual/contextual.libraries.yml
+++ b/core/modules/contextual/contextual.libraries.yml
@@ -3,14 +3,14 @@ drupal.contextual-links:
   js:
     # Ensure to run before contextual/drupal.context-toolbar.
     # Core.
-    js/contextual.js: { weight: -2 }
+    js/contextual.js: {}
     # Models.
-    js/models/StateModel.js: { weight: -2 }
+    js/models/StateModel.js: {}
     # Views.
-    js/views/AuralView.js: { weight: -2 }
-    js/views/KeyboardView.js: { weight: -2 }
-    js/views/RegionView.js: { weight: -2 }
-    js/views/VisualView.js: { weight: -2 }
+    js/views/AuralView.js: {}
+    js/views/KeyboardView.js: {}
+    js/views/RegionView.js: {}
+    js/views/VisualView.js: {}
   css:
     component:
       css/contextual.module.css: {}
diff --git a/core/modules/simpletest/simpletest.module b/core/modules/simpletest/simpletest.module
index 052e0b7..55e62af 100644
--- a/core/modules/simpletest/simpletest.module
+++ b/core/modules/simpletest/simpletest.module
@@ -49,18 +49,6 @@ function simpletest_theme() {
 }
 
 /**
- * Implements hook_js_alter().
- */
-function simpletest_js_alter(&$javascript, AttachedAssetsInterface $assets) {
-  // Since SimpleTest is a special use case for the table select, stick the
-  // SimpleTest JavaScript above the table select.
-  $simpletest = drupal_get_path('module', 'simpletest') . '/simpletest.js';
-  if (array_key_exists($simpletest, $javascript) && array_key_exists('core/misc/tableselect.js', $javascript)) {
-    $javascript[$simpletest]['weight'] = $javascript['core/misc/tableselect.js']['weight'] - 1;
-  }
-}
-
-/**
  * Prepares variables for simpletest result summary templates.
  *
  * Default template: simpletest-result-summary.html.twig.
diff --git a/core/modules/system/src/Tests/Common/AttachedAssetsTest.php b/core/modules/system/src/Tests/Common/AttachedAssetsTest.php
index 10bf818..8098fdd 100644
--- a/core/modules/system/src/Tests/Common/AttachedAssetsTest.php
+++ b/core/modules/system/src/Tests/Common/AttachedAssetsTest.php
@@ -389,21 +389,23 @@ function testRenderDifferentWeight() {
   /**
    * Tests altering a JavaScript's weight via hook_js_alter().
    *
-   * @see simpletest_js_alter()
+   * @see attached_assets_test_js_alter()
    */
   function testAlter() {
-    // Add both tableselect.js and simpletest.js.
-    $build['#attached']['library'][] = 'core/drupal.tableselect';
-    $build['#attached']['library'][] = 'simpletest/drupal.simpletest';
+    // Enable a test module that alters JS.
+    $this->enableModules(['attached_assets_test']);
+
+    // Add jQuery.
+    $build['#attached']['library'][] = 'core/jquery';
     $assets = AttachedAssets::createFromRenderArray($build);
 
-    // Render the JavaScript, testing if simpletest.js was altered to be before
-    // tableselect.js. See simpletest_js_alter() to see where this alteration
-    // takes place.
+    // Render the JavaScript, testing if attached_assets_test.js was added. See
+    // attached_assets_test_js_alter() to see this file being added using
+    // hook_js_alter().
     $js = $this->assetResolver->getJsAssets($assets, FALSE)[1];
     $js_render_array = \Drupal::service('asset.js.collection_renderer')->render($js);
     $rendered_js = $this->renderer->render($js_render_array);
-    $this->assertTrue(strpos($rendered_js, 'simpletest.js') < strpos($rendered_js, 'core/misc/tableselect.js'), 'Altering JavaScript weight through the alter hook.');
+    $this->assertTrue(strpos($rendered_js, 'attached_assets_test.js') > 0, 'Altering JavaScript weight through the alter hook.');
   }
 
   /**
diff --git a/core/modules/system/system.libraries.yml b/core/modules/system/system.libraries.yml
index 34c90dd..20bdb64 100644
--- a/core/modules/system/system.libraries.yml
+++ b/core/modules/system/system.libraries.yml
@@ -3,9 +3,9 @@ base:
   css:
     # Adjust the weights to load these early.
     component:
-      css/system.module.css: { every_page: true, weight: -10, }
+      css/system.module.css: { every_page: true }
     theme:
-      css/system.theme.css: { every_page: true, weight: -10 }
+      css/system.theme.css: { every_page: true }
   dependencies:
     - core/normalize
 
@@ -13,7 +13,7 @@ admin:
   version: VERSION
   css:
     theme:
-      css/system.admin.css: { weight: -10 }
+      css/system.admin.css: {}
   dependencies:
     - system/base
 
@@ -21,7 +21,7 @@ maintenance:
   version: VERSION
   css:
     theme:
-      css/system.maintenance.css: { weight: -10 }
+      css/system.maintenance.css: {}
   dependencies:
     - system/base
     - system/admin
diff --git a/core/modules/system/tests/modules/attached_assets_test/attached_assets_test.info.yml b/core/modules/system/tests/modules/attached_assets_test/attached_assets_test.info.yml
new file mode 100644
index 0000000..c6b147d
--- /dev/null
+++ b/core/modules/system/tests/modules/attached_assets_test/attached_assets_test.info.yml
@@ -0,0 +1,6 @@
+name: 'Attached assets mock module'
+type: module
+description: 'Test for attached asset calls.'
+core: 8.x
+package: Testing
+version: VERSION
diff --git a/core/modules/system/tests/modules/attached_assets_test/attached_assets_test.js b/core/modules/system/tests/modules/attached_assets_test/attached_assets_test.js
new file mode 100644
index 0000000..6a78081c5
--- /dev/null
+++ b/core/modules/system/tests/modules/attached_assets_test/attached_assets_test.js
@@ -0,0 +1,4 @@
+/**
+ * @file
+ * Empty JavaScript file to test attached assets.
+ */
diff --git a/core/modules/system/tests/modules/attached_assets_test/attached_assets_test.module b/core/modules/system/tests/modules/attached_assets_test/attached_assets_test.module
new file mode 100644
index 0000000..adf41d0
--- /dev/null
+++ b/core/modules/system/tests/modules/attached_assets_test/attached_assets_test.module
@@ -0,0 +1,17 @@
+<?php
+
+/**
+ * @file
+ * Simpletest mock module for Attached Assets testing.
+ */
+
+use Drupal\Core\Asset\AttachedAssetsInterface;
+
+/**
+ * Attached Assets Test: hook_js_alter().
+ */
+function attached_assets_test_js_alter(&$javascript, AttachedAssetsInterface $assets) {
+  $file = drupal_get_path('module', 'attached_assets_test') . '/attached_assets_test.js';
+  $javascript[$file] = $javascript['core/assets/vendor/jquery/jquery.min.js'];
+  $javascript[$file]['data'] = $file;
+}
diff --git a/core/modules/views/views.libraries.yml b/core/modules/views/views.libraries.yml
index 03efb2c..40bfc4c 100644
--- a/core/modules/views/views.libraries.yml
+++ b/core/modules/views/views.libraries.yml
@@ -21,7 +21,7 @@ views.contextual-links:
   version: VERSION
   js:
     # Ensure to run before contextual/drupal.contextual-links.
-    js/views-contextual.js: { weight: -10 }
+    js/views-contextual.js: {}
   dependencies:
     - core/jquery
     - core/drupal