diff --git a/core/lib/Drupal/Core/Composer/Dependencies/DevDependencyFinder.php b/core/lib/Drupal/Core/Composer/Dependencies/DevDependencyFinder.php
index 7ccc470257..97df2e16df 100644
--- a/core/lib/Drupal/Core/Composer/Dependencies/DevDependencyFinder.php
+++ b/core/lib/Drupal/Core/Composer/Dependencies/DevDependencyFinder.php
@@ -53,13 +53,7 @@ public function buildRequirements() {
     $dev_requirements = $this->projectRoot->getDevRequirements();
 
     // Make a list of installed dev requirements.
-    $installed_dev = [];
-    $installed_package_names = array_keys($installed);
-    foreach (array_keys($dev_requirements) as $dev_requirement_name) {
-      if (in_array($dev_requirement_name, $installed_package_names)) {
-        $installed_dev[$dev_requirement_name] = $installed[$dev_requirement_name];
-      }
-    }
+    $installed_dev = array_intersect_key($installed, $dev_requirements);
 
     // If the number of dev requirements and installed dev requirements is
     // exactly the same, then we reason that the user probably issued a
@@ -75,12 +69,12 @@ public function buildRequirements() {
     // for core's dev requirements. And we're unable to tell the difference
     // anyway.
     $dev_installed = FALSE;
-    if (!empty($installed_dev) && (count($installed_dev) === count($dev_requirements))) {
+    if ($installed_dev && (count($installed_dev) === count($dev_requirements))) {
       $dev_installed = TRUE;
       foreach ($dev_requirements as $package_name => $version_constraint) {
         // Special case for @dev packages, which can generate unparseable local
         // version numbers, such as [behat/mink] => 9999999-dev.
-        if (!strpos($version_constraint, 'dev')) {
+        if (strpos($version_constraint, 'dev') === FALSE) {
           if (!Semver::satisfies($installed_dev[$package_name], $version_constraint)) {
             $dev_installed = FALSE;
             break;
diff --git a/core/lib/Drupal/Core/Composer/ProjectRoot.php b/core/lib/Drupal/Core/Composer/ProjectRoot.php
index 930ad6621d..1ffd2a3cc9 100644
--- a/core/lib/Drupal/Core/Composer/ProjectRoot.php
+++ b/core/lib/Drupal/Core/Composer/ProjectRoot.php
@@ -40,6 +40,11 @@ public function __construct($app_root) {
    */
   public function getProjectVendorDirectory() {
     if (empty($this->vendorDirectory)) {
+      // In order to discover the vendor directory, we require the Drupal class
+      // loader facade which is always in appRoot. appRoot/autoload.php
+      // is supposed to pass through the actual class loader, wherever it might
+      // be in the file system. Once we have this class loader object, we can
+      // use reflection to deduce the location of vendor/.
       $vendor_dir = $this->appRoot . '/vendor';
       $autoload = require $this->appRoot . '/autoload.php';
       if ($autoload instanceof \Composer\Autoload\ClassLoader) {
diff --git a/core/lib/Drupal/Core/Composer/ProjectRootInterface.php b/core/lib/Drupal/Core/Composer/ProjectRootInterface.php
index 0e9ec8b2e6..32dc53cb3f 100644
--- a/core/lib/Drupal/Core/Composer/ProjectRootInterface.php
+++ b/core/lib/Drupal/Core/Composer/ProjectRootInterface.php
@@ -4,9 +4,6 @@
 
 /**
  * Provides information about the Composer project root.
- *
- * @todo Some of this would very well be changed in
- *   https://www.drupal.org/node/2494073
  */
 interface ProjectRootInterface {
 
@@ -36,7 +33,7 @@ public function getInstalledPackages();
    * will return a combination of the root package and the drupal/core package.
    *
    * This method only deals with core's dev requirements, and does not include
-   * dev requirements for contrib. It's likely that if core and contrib's dev
+   * dev requirements for contrib. It's likely that core and contrib's dev
    * requirements are similar.
    *
    * @return string[]
diff --git a/core/tests/Drupal/KernelTests/Core/Composer/Dependencies/DevDependencyFinderTest.php b/core/tests/Drupal/KernelTests/Core/Composer/Dependencies/DevDependencyFinderTest.php
index d1ad5b4a3c..49e67844ad 100644
--- a/core/tests/Drupal/KernelTests/Core/Composer/Dependencies/DevDependencyFinderTest.php
+++ b/core/tests/Drupal/KernelTests/Core/Composer/Dependencies/DevDependencyFinderTest.php
@@ -16,6 +16,7 @@
  *
  * @todo Make this a UnitTestBase test when the systems it tests no longer have
  *   dependencies on install.inc.
+ * @see https://www.drupal.org/node/2909480
  */
 class DevDependencyFinderTest extends KernelTestBase {