Getting Kernel tests working post immutable context

I have plausible solutions for the first two items, as I've said before.

Getting a context to wrap a NULL value, however, appears to be hard to do. I've tried to create a sort of "typed null" object (an 'any', setting the value to NULL), but Context's implementation of getContextValue() will throw an except:

  /**
   * {@inheritdoc}
   */
  public function getContextValue() {
    if (!isset($this->contextData)) {
      $definition = $this->getContextDefinition();
      $default_value = $definition->getDefaultValue();

      if (isset($default_value)) {
        // Keep the default value here so that subsequent calls don't have to
        // look it up again.
        $this->setContextValue($default_value);
      }
      elseif ($definition->isRequired()) {
        $type = $definition->getDataType();
        throw new ContextException("The '$type' context is required and not present.");
      }
      return $default_value;
    }
    return $this->getTypedDataManager()->getCanonicalRepresentation($this->contextData);
  }

I don't see a good way around this. Worse: I'm seeing some weird behavior even when it doesn't throw.

I'm going to post patches for my approach to the other two issues, since I don't think it's to the point I can create a PR that's worth posting.

The patch shows changes to ContextHandlerTrait and RulesState that resolve 14 out of the 15 issues in the Kernel tests. Again, not sure that the behavior is what you want, but at least the tests like them.

@emclaughlin: take a look at #2573823: Context in Rules plugins does not respect annotation order. Some of the problems I've seen related to doExecute() getting its arguments in the wrong order, which can happen with default arguments right now.

Is this relevant to your problem?

See #2575977: Major change in \Drupal\Core\Plugin\Context\Context breaks Rules and related issues for details on the fix.