Determine which config entities can be fixed and which will be deleted when a dependency is removed

This will allow for significantly enhanced usability/DX. The code looks beautiful and promising. :)

1. +++ b/core/lib/Drupal/Core/Config/ConfigManager.php
   @@ -311,6 +321,68 @@ public function findConfigEntityDependentsAsEntities($type, array $names) {
   +      'theme' => []
   

   Should have a trailing comma.

2. +++ b/core/lib/Drupal/Core/Config/ConfigManager.php
   @@ -311,6 +321,68 @@ public function findConfigEntityDependentsAsEntities($type, array $names) {
   +    // Try to fix any dependencies and find out what will happen to the
   +    // dependency graph.
   

   That sounds beautiful :)

   The body of the loop is a bit hard to follow for those less familiar with details of CMI though (at least for me). A few more high-level comments at for the three if-branches would be helpful.

3. +++ b/core/lib/Drupal/Core/Config/ConfigManager.php
   @@ -311,6 +321,68 @@ public function findConfigEntityDependentsAsEntities($type, array $names) {
   +      if (empty($dependents) || reset($dependents)->uuid() == $first_uuid) {
   +        // We've looped around everything or all dependencies are fixable.
   

   I don't think the empty check is necessary, the while condition takes care of that?

+++ b/core/lib/Drupal/Core/Config/ConfigManager.php
@@ -311,6 +287,69 @@ public function findConfigEntityDependentsAsEntities($type, array $names) {
+  public function getConfigEntitiesToChangeOnDependencyRemoval($type, array $names, $dry_run = TRUE) {

+1 for the function name.

@alexpott I wasn't judging! I agree with everything you said.

#4: Just listing which entities is affected on the module uninstall confirm form is not that helpful for users.
+1

#7: […] that's why it is good that everything - config uninstall and config entity delete will use the same code path. HURRAY!

#12: I think the only way to shorten the name is to have an intermediate value object. Which is probably not justified here.

I can basically only find nitpicks:

1. +++ b/core/lib/Drupal/Core/Config/ConfigManagerInterface.php
   @@ -126,6 +136,30 @@ public function findConfigEntityDependents($type, array $names);
   +   * @param bool $dry_run
   +   *   If set to FALSE the entities returned in the list of updates will
   +   *   modified. In order to make the changes the caller needs to save them. If
   +   *   set to TRUE the entities returned will not be modified.
   

   s/will modified/will be modified/

2. +++ b/core/lib/Drupal/Core/Config/ConfigManagerInterface.php
   @@ -126,6 +136,30 @@ public function findConfigEntityDependents($type, array $names);
   +   *   processed before deletes. The order of the deletes is significant and
   +   *   must be processed in the order.
   

   "in the returned order" ?

3. +++ b/core/lib/Drupal/Core/Config/Entity/ConfigDependencyManager.php
   @@ -286,4 +286,20 @@ public function setData(array $data) {
   +   * @param $dependencies
   ...
   +  public function updateData($name, $dependencies) {
   

   Nit: Typehint to array?

4. +++ b/core/modules/system/src/Form/ModulesUninstallConfirmForm.php
   @@ -145,43 +145,83 @@ public function buildForm(array $form, FormStateInterface $form_state) {
   +      '#title' => $this->t('Configuration deletes'),
   

   "deletes" or "deletions"?

Just one tiny remaining nit that can be fixed on commit: the #14.3 typehint is added to the code, but missing in the docs.

+++ b/core/lib/Drupal/Core/Config/ConfigManager.php
@@ -311,6 +287,69 @@ public function findConfigEntityDependentsAsEntities($type, array $names) {
+        // Ensure that the dependency has actually been fixed. it is possible

Just fixed missing capital letter here on commit.

Fantastic patch. Committed/pushed to 8.0.x, thanks!