diff --git a/core/lib/Drupal/Core/Entity/EntityManager.php b/core/lib/Drupal/Core/Entity/EntityManager.php
index 18061b9..29b0d31 100644
--- a/core/lib/Drupal/Core/Entity/EntityManager.php
+++ b/core/lib/Drupal/Core/Entity/EntityManager.php
@@ -349,10 +349,10 @@ protected function buildBaseFieldDefinitions($entity_type_id) {
       if (!empty($module_definitions)) {
         // Ensure the provider key actually matches the name of the provider
         // defining the field.
-        foreach ($module_definitions as $definition) {
+        foreach ($module_definitions as $field_name => $definition) {
           $definition->setProvider($module);
+          $base_field_definitions[$field_name] = $definition;
         }
-        $base_field_definitions = NestedArray::mergeDeep($base_field_definitions, $module_definitions);
       }
     }
 
@@ -436,10 +436,10 @@ protected function buildBundleFieldDefinitions($entity_type_id, $bundle, array $
       if (!empty($module_definitions)) {
         // Ensure the provider key actually matches the name of the provider
         // defining the field.
-        foreach ($module_definitions as $definition) {
+        foreach ($module_definitions as $field_name => $definition) {
           $definition->setProvider($module);
+          $bundle_field_definitions[$field_name] = $definition;
         }
-        $bundle_field_definitions = NestedArray::mergeDeep($bundle_field_definitions, $module_definitions);
       }
     }
 
diff --git a/core/lib/Drupal/Core/Field/FieldDefinition.php b/core/lib/Drupal/Core/Field/FieldDefinition.php
index 5321836..6cfec74 100644
--- a/core/lib/Drupal/Core/Field/FieldDefinition.php
+++ b/core/lib/Drupal/Core/Field/FieldDefinition.php
@@ -156,7 +156,12 @@ public function getProvider() {
   }
 
   /**
-   * {@inheritdoc}
+   * Sets the name of the provider of this field.
+   *
+   * @param string $provider
+   *   The provider name to set.
+   *
+   * @return $this
    */
   public function setProvider($provider) {
     $this->definition['provider'] = $provider;
@@ -472,8 +477,8 @@ public function hasCustomStorage() {
    * Sets the storage behavior for this field.
    *
    * @param bool $custom_storage
-   *   Pass TRUE if the field module will take care of storing field values,
-   *   FALSE otherwise.
+   *   Pass FALSE if the storage controller takes care of storing the field,
+   *   TRUE otherwise.
    *
    * @return $this
    */
diff --git a/core/lib/Drupal/Core/Field/FieldDefinitionInterface.php b/core/lib/Drupal/Core/Field/FieldDefinitionInterface.php
index 966ba8e..b0eab20 100644
--- a/core/lib/Drupal/Core/Field/FieldDefinitionInterface.php
+++ b/core/lib/Drupal/Core/Field/FieldDefinitionInterface.php
@@ -107,21 +107,11 @@ public function getSetting($setting_name);
    * Returns the name of the provider of this field.
    *
    * @return string
-   *   The module name.
+   *   The provider name; e.g., the module name.
    */
   public function getProvider();
 
   /**
-   * Sets the name of the provider of this field.
-   *
-   * @param string $provider
-   *   The provider name.
-   *
-   * @return $this
-   */
-  public function setProvider($provider);
-
-  /**
    * Returns whether the field is translatable.
    *
    * @return bool
@@ -350,11 +340,11 @@ public function getColumns();
    * Returns the storage behavior for this field.
    *
    * Indicates whether the entity type's storage controller should take care of
-   * storing the field values or whether the module providing the field will do
-   * that.
+   * storing the field values or whether it is handled separately; e.g. by the
+   * module providing the field.
    *
    * @return bool
-   *   TRUE if the field module takes care of storing field values, FALSE
+   *   FALSE if the storage controller takes care of storing the field, TRUE
    *   otherwise.
    */
   public function hasCustomStorage();
diff --git a/core/modules/field/lib/Drupal/field/Entity/FieldConfig.php b/core/modules/field/lib/Drupal/field/Entity/FieldConfig.php
index 545ef86..971e246 100644
--- a/core/modules/field/lib/Drupal/field/Entity/FieldConfig.php
+++ b/core/modules/field/lib/Drupal/field/Entity/FieldConfig.php
@@ -590,14 +590,6 @@ public function getProvider() {
   /**
    * {@inheritdoc}
    */
-  public function setProvider($provider) {
-    // Do nothing: provider is a readonly property here.
-    return $this;
-  }
-
-  /**
-   * {@inheritdoc}
-   */
   public function getLabel() {
     return $this->label();
   }
diff --git a/core/modules/field/lib/Drupal/field/Entity/FieldInstanceConfig.php b/core/modules/field/lib/Drupal/field/Entity/FieldInstanceConfig.php
index 0bf3a92..e249355 100644
--- a/core/modules/field/lib/Drupal/field/Entity/FieldInstanceConfig.php
+++ b/core/modules/field/lib/Drupal/field/Entity/FieldInstanceConfig.php
@@ -508,14 +508,6 @@ public function getProvider() {
   /**
    * {@inheritdoc}
    */
-  public function setProvider($provider) {
-    $this->field->setProvider($provider);
-    return $this;
-  }
-
-  /**
-   * {@inheritdoc}
-   */
   public function isTranslatable() {
     return $this->field->translatable;
   }