gistfile1.php

<?php

// Adding a method to TypedDataManager

  public function mapDefinition(array $type) {
    /** $type comes across as:
     * 
     * "type" = "entity",
     * "constraints" = {
     *   "EntityType" = "node"
     * }
     * 
     */
    
    $definition = new DataDefinition($this->getDefinition($type['type']));
    foreach ($type['constraints'] as $constraint_type => $value) {
      $definition->addConstraint($constraint_type, $value);
    }
    return $definition;
    
  }

gistfile2.php

<?php

// Altering addConstraint within DataDefinition & co to something like so:

public function addConstraint($constraint_name, $options = NULL) {
  $constraint_definition = $this->constraintManager->getDefinition($constraint_name);
  if (!isset($this->definition['constraints'][$constraint_name])) {
    $this->definition['constraints'][$constraint_name] = $options;
  }
  elseif ($constraint_definition->supportMultiple()) {
    $this->definition['constraints'][$constraint_name] = $options;
  }
  else {
    throw new ConstraintException("Nuh uh");
  }
  // This constraint may need to alter the data type.
  $new_type = $constraint_definition->alterDataType($this->getDataType());
  if ($new_type != $this->getDataType()) {
    $type_data_definition = \Drupal::service('typed_data')->getDefinition($new_type);
    // This will add the constraints during construction.
    $definition = new DataDefinition($typed_data_definition + $this->definition);
    $definition->setDataType($new_type);
    return $definition;
  }
  return $this;
}