.../EntityResource/HalEntityNormalizationTrait.php    | 14 ++++++--------
 .../EntityResource/Block/BlockResourceTestBase.php    |  6 +++++-
 .../Comment/CommentResourceTestBase.php               |  6 +++++-
 .../ConfigTest/ConfigTestResourceTestBase.php         |  6 +++++-
 .../EntityTest/EntityTestResourceTestBase.php         |  6 +++++-
 .../EntityResource/Node/NodeResourceTestBase.php      |  6 +++++-
 .../EntityResource/Role/RoleResourceTestBase.php      |  6 +++++-
 .../EntityResource/Term/TermResourceTestBase.php      |  6 +++++-
 .../EntityResource/User/UserResourceTestBase.php      |  6 +++++-
 .../Vocabulary/VocabularyResourceTestBase.php         |  6 +++++-
 .../rest/tests/src/Functional/ResourceTestBase.php    | 19 +++++++------------
 11 files changed, 58 insertions(+), 29 deletions(-)

diff --git a/core/modules/hal/tests/src/Functional/EntityResource/HalEntityNormalizationTrait.php b/core/modules/hal/tests/src/Functional/EntityResource/HalEntityNormalizationTrait.php
index 49ac07d..ade5239 100644
--- a/core/modules/hal/tests/src/Functional/EntityResource/HalEntityNormalizationTrait.php
+++ b/core/modules/hal/tests/src/Functional/EntityResource/HalEntityNormalizationTrait.php
@@ -27,10 +27,7 @@ protected function applyHalFieldNormalization(array $normalization) {
     // bundle field.
     $bundle_key = $this->entity->getEntityType()->getKey('bundle');
     $reference_fields = array_keys(array_filter($this->entity->getFields(), function (FieldItemListInterface $field) use ($bundle_key) {
-      if ($field->getName() === $bundle_key) {
-        return FALSE;
-      }
-      return $field instanceof EntityReferenceFieldItemListInterface;
+      return ($field->getName() === $bundle_key) ? FALSE : $field instanceof EntityReferenceFieldItemListInterface;
     }));
     foreach ($reference_fields as $field_name) {
       unset($normalization[$field_name]);
@@ -44,10 +41,11 @@ protected function applyHalFieldNormalization(array $normalization) {
     }
 
     // In the HAL normalization, empty fields are omitted.
-    foreach ($normalization as $field_name => $data) {
-      if ($this->entity->$field_name->isEmpty()) {
-        unset($normalization[$field_name]);
-      }
+    $empty_fields = array_keys(array_filter($this->entity->getFields(), function (FieldItemListInterface $field) {
+      return $field->isEmpty();
+    }));
+    foreach ($empty_fields as $field_name) {
+      unset($normalization[$field_name]);
     }
 
     return $normalization;
diff --git a/core/modules/rest/tests/src/Functional/EntityResource/Block/BlockResourceTestBase.php b/core/modules/rest/tests/src/Functional/EntityResource/Block/BlockResourceTestBase.php
index 268f690..c569c20 100644
--- a/core/modules/rest/tests/src/Functional/EntityResource/Block/BlockResourceTestBase.php
+++ b/core/modules/rest/tests/src/Functional/EntityResource/Block/BlockResourceTestBase.php
@@ -18,6 +18,11 @@
   protected static $entityType = 'block';
 
   /**
+   * @var \Drupal\block\BlockInterface
+   */
+  protected $entity;
+
+  /**
    * {@inheritdoc}
    */
   protected function setUpAuthorization($method) {
@@ -69,7 +74,6 @@ protected function createEntity() {
    * {@inheritdoc}
    */
   protected function getExpectedNormalizedEntity() {
-    assert('$this->entity instanceof \Drupal\Core\Entity\EntityInterface', 'Entity must already have been created.');
     $normalization = [
       'uuid' => $this->entity->uuid(),
       'id' => 'llama',
diff --git a/core/modules/rest/tests/src/Functional/EntityResource/Comment/CommentResourceTestBase.php b/core/modules/rest/tests/src/Functional/EntityResource/Comment/CommentResourceTestBase.php
index d602524..ef37d1b 100644
--- a/core/modules/rest/tests/src/Functional/EntityResource/Comment/CommentResourceTestBase.php
+++ b/core/modules/rest/tests/src/Functional/EntityResource/Comment/CommentResourceTestBase.php
@@ -25,6 +25,11 @@
   protected static $entityType = 'comment';
 
   /**
+   * @var \Drupal\comment\CommentInterface
+   */
+  protected $entity;
+
+  /**
    * {@inheritdoc}
    */
   protected function setUpAuthorization($method) {
@@ -83,7 +88,6 @@ protected function createEntity() {
    * {@inheritdoc}
    */
   protected function getExpectedNormalizedEntity() {
-    assert('$this->entity instanceof \Drupal\Core\Entity\EntityInterface', 'Entity must already have been created.');
     $author = User::load(0);
     return [
       'cid' => [
diff --git a/core/modules/rest/tests/src/Functional/EntityResource/ConfigTest/ConfigTestResourceTestBase.php b/core/modules/rest/tests/src/Functional/EntityResource/ConfigTest/ConfigTestResourceTestBase.php
index 9024eae..50c6ec9 100644
--- a/core/modules/rest/tests/src/Functional/EntityResource/ConfigTest/ConfigTestResourceTestBase.php
+++ b/core/modules/rest/tests/src/Functional/EntityResource/ConfigTest/ConfigTestResourceTestBase.php
@@ -17,6 +17,11 @@
   protected static $entityType = 'config_test';
 
   /**
+   * @var \Drupal\config_test\ConfigTestInterface
+   */
+  protected $entity;
+
+  /**
    * {@inheritdoc}
    */
   protected function setUpAuthorization($method) {
@@ -40,7 +45,6 @@ protected function createEntity() {
    * {@inheritdoc}
    */
   protected function getExpectedNormalizedEntity() {
-    assert('$this->entity instanceof \Drupal\Core\Entity\EntityInterface', 'Entity must already have been created.');
     $normalization = [
       'uuid' => $this->entity->uuid(),
       'id' => 'llama',
diff --git a/core/modules/rest/tests/src/Functional/EntityResource/EntityTest/EntityTestResourceTestBase.php b/core/modules/rest/tests/src/Functional/EntityResource/EntityTest/EntityTestResourceTestBase.php
index 5dff569..ab375f9 100644
--- a/core/modules/rest/tests/src/Functional/EntityResource/EntityTest/EntityTestResourceTestBase.php
+++ b/core/modules/rest/tests/src/Functional/EntityResource/EntityTest/EntityTestResourceTestBase.php
@@ -19,6 +19,11 @@
   protected static $entityType = 'entity_test';
 
   /**
+   * @var \Drupal\entity_test\Entity\EntityTest
+   */
+  protected $entity;
+
+  /**
    * {@inheritdoc}
    */
   protected function setUpAuthorization($method) {
@@ -54,7 +59,6 @@ protected function createEntity() {
    * {@inheritdoc}
    */
   protected function getExpectedNormalizedEntity() {
-    assert('$this->entity instanceof \Drupal\Core\Entity\EntityInterface', 'Entity must already have been created.');
     $author = User::load(0);
     $normalization = [
       'uuid' => [
diff --git a/core/modules/rest/tests/src/Functional/EntityResource/Node/NodeResourceTestBase.php b/core/modules/rest/tests/src/Functional/EntityResource/Node/NodeResourceTestBase.php
index ba1ec18..e77467a 100644
--- a/core/modules/rest/tests/src/Functional/EntityResource/Node/NodeResourceTestBase.php
+++ b/core/modules/rest/tests/src/Functional/EntityResource/Node/NodeResourceTestBase.php
@@ -20,6 +20,11 @@
   protected static $entityType = 'node';
 
   /**
+   * @var \Drupal\node\NodeInterface
+   */
+  protected $entity;
+
+  /**
    * {@inheritdoc}
    */
   protected function setUpAuthorization($method) {
@@ -67,7 +72,6 @@ protected function createEntity() {
    * {@inheritdoc}
    */
   protected function getExpectedNormalizedEntity() {
-    assert('$this->entity instanceof \Drupal\Core\Entity\EntityInterface', 'Entity must already have been created.');
     $author = User::load(0);
     return [
       'nid' => [
diff --git a/core/modules/rest/tests/src/Functional/EntityResource/Role/RoleResourceTestBase.php b/core/modules/rest/tests/src/Functional/EntityResource/Role/RoleResourceTestBase.php
index 12f329c..4cc247c 100644
--- a/core/modules/rest/tests/src/Functional/EntityResource/Role/RoleResourceTestBase.php
+++ b/core/modules/rest/tests/src/Functional/EntityResource/Role/RoleResourceTestBase.php
@@ -19,6 +19,11 @@
   protected static $entityType = 'user_role';
 
   /**
+   * @var \Drupal\user\RoleInterface
+   */
+  protected $entity;
+
+  /**
    * {@inheritdoc}
    */
   protected function setUpAuthorization($method) {
@@ -42,7 +47,6 @@ protected function createEntity() {
    * {@inheritdoc}
    */
   protected function getExpectedNormalizedEntity() {
-    assert('$this->entity instanceof \Drupal\Core\Entity\EntityInterface', 'Entity must already have been created.');
     return [
       'uuid' => $this->entity->uuid(),
       'weight' => 2,
diff --git a/core/modules/rest/tests/src/Functional/EntityResource/Term/TermResourceTestBase.php b/core/modules/rest/tests/src/Functional/EntityResource/Term/TermResourceTestBase.php
index 7c9f0f2..dd56380 100644
--- a/core/modules/rest/tests/src/Functional/EntityResource/Term/TermResourceTestBase.php
+++ b/core/modules/rest/tests/src/Functional/EntityResource/Term/TermResourceTestBase.php
@@ -19,6 +19,11 @@
   protected static $entityType = 'taxonomy_term';
 
   /**
+   * @var \Drupal\taxonomy\TermInterface
+   */
+  protected $entity;
+
+  /**
    * {@inheritdoc}
    */
   protected function setUpAuthorization($method) {
@@ -62,7 +67,6 @@ protected function createEntity() {
    * {@inheritdoc}
    */
   protected function getExpectedNormalizedEntity() {
-    assert('$this->entity instanceof \Drupal\Core\Entity\EntityInterface', 'Entity must already have been created.');
     return [
       'tid' => [
         ['value' => 1],
diff --git a/core/modules/rest/tests/src/Functional/EntityResource/User/UserResourceTestBase.php b/core/modules/rest/tests/src/Functional/EntityResource/User/UserResourceTestBase.php
index fa6f32c..478c442 100644
--- a/core/modules/rest/tests/src/Functional/EntityResource/User/UserResourceTestBase.php
+++ b/core/modules/rest/tests/src/Functional/EntityResource/User/UserResourceTestBase.php
@@ -18,6 +18,11 @@
   protected static $entityType = 'user';
 
   /**
+   * @var \Drupal\user\UserInterface
+   */
+  protected $entity;
+
+  /**
    * {@inheritdoc}
    */
   protected static $labelField = 'name';
@@ -66,7 +71,6 @@ protected function createEntity() {
    * {@inheritdoc}
    */
   protected function getExpectedNormalizedEntity() {
-    assert('$this->entity instanceof \Drupal\Core\Entity\EntityInterface', 'Entity must already have been created.');
     return [
       'uid' => [
         ['value' => '3'],
diff --git a/core/modules/rest/tests/src/Functional/EntityResource/Vocabulary/VocabularyResourceTestBase.php b/core/modules/rest/tests/src/Functional/EntityResource/Vocabulary/VocabularyResourceTestBase.php
index 241eb17..e812fbb 100644
--- a/core/modules/rest/tests/src/Functional/EntityResource/Vocabulary/VocabularyResourceTestBase.php
+++ b/core/modules/rest/tests/src/Functional/EntityResource/Vocabulary/VocabularyResourceTestBase.php
@@ -21,6 +21,11 @@
   protected static $entityType = 'taxonomy_vocabulary';
 
   /**
+   * @var \Drupal\taxonomy\VocabularyInterface
+   */
+  protected $entity;
+
+  /**
    * {@inheritdoc}
    */
   protected function setUpAuthorization($method) {
@@ -44,7 +49,6 @@ protected function createEntity() {
    * {@inheritdoc}
    */
   protected function getExpectedNormalizedEntity() {
-    assert('$this->entity instanceof \Drupal\Core\Entity\EntityInterface', 'Entity must already have been created.');
     return [
       'uuid' => $this->entity->uuid(),
       'vid' => 'llama',
diff --git a/core/modules/rest/tests/src/Functional/ResourceTestBase.php b/core/modules/rest/tests/src/Functional/ResourceTestBase.php
index 07cf862..549ebad 100644
--- a/core/modules/rest/tests/src/Functional/ResourceTestBase.php
+++ b/core/modules/rest/tests/src/Functional/ResourceTestBase.php
@@ -9,6 +9,7 @@
 use Drupal\user\RoleInterface;
 use GuzzleHttp\Exception\ClientException;
 use GuzzleHttp\Exception\ServerException;
+use GuzzleHttp\RequestOptions;
 use Psr\Http\Message\ResponseInterface;
 
 /**
@@ -261,8 +262,10 @@ protected function grantPermissionsToTestedRole(array $permissions) {
   /**
    * Performs a HTTP request. Wraps the Guzzle HTTP client.
    *
-   * Why wrap the Guzzle HTTP client? Because any error response is returned via
-   * an exception, which would make the tests unnecessarily complex to read.
+   * Why wrap the Guzzle HTTP client? Because we want to keep the actual test
+   * code as simple as possible, and hence not require them to specify the
+   * 'http_errors = FALSE' request option, nor do we want them to have to
+   * convert Drupal Url objects to strings.
    *
    * @see \GuzzleHttp\ClientInterface::request()
    *
@@ -276,16 +279,8 @@ protected function grantPermissionsToTestedRole(array $permissions) {
    * @return \Psr\Http\Message\ResponseInterface
    */
   protected function request($method, Url $url, array $request_options) {
-    try {
-      $response = $this->httpClient->request($method, $url->toString(), $request_options);
-    }
-    catch (ClientException $e) {
-      $response = $e->getResponse();
-    }
-    catch (ServerException $e) {
-      $response = $e->getResponse();
-    }
-    return $response;
+    $request_options[RequestOptions::HTTP_ERRORS] = FALSE;
+    return $this->httpClient->request($method, $url->toString(), $request_options);
   }
 
   /**