Wikimedia\Composer\Merge\PluginState::isDevMode PHP Method

    public function isDevMode()
    {
        return $this->shouldMergeDev() && $this->devMode;
    }

 /**
  * Handle an event callback for pre-dependency solving phase of an install
  * or update by adding any duplicate package dependencies found during
  * initial merge processing to the request that will be processed by the
  * dependency solver.
  *
  * @param InstallerEvent $event
  */
 public function onDependencySolve(InstallerEvent $event)
 {
     $request = $event->getRequest();
     foreach ($this->state->getDuplicateLinks('require') as $link) {
         $this->logger->info("Adding dependency <comment>{$link}</comment>");
         $request->install($link->getTarget(), $link->getConstraint());
     }
     if ($this->state->isDevMode()) {
         foreach ($this->state->getDuplicateLinks('require-dev') as $link) {
             $this->logger->info("Adding dev dependency <comment>{$link}</comment>");
             $request->install($link->getTarget(), $link->getConstraint());
         }
     }
 }