Add the Symfony var-dumper component as a dependency

I guess Symfony RequestDataCollector doesn't declare this dependency?

In a Symfony app it is required by the symfony/framework-bundle package that we doesn't use in Drupal

I'm OK with adding it, but I pause with the 3.4.0 constraint. Some folks may want to run the new devel with an earlier version of Drupal? Was Drupal 8.4 using Symfony 3.4?

Drupal 8.4 was using 3.2. Maybe we can use the same dependency constraint as drush: "symfony/var-dumper": "~2.7|^3" ?

Committed and pushed to 8.x-1.x

Of course Webprofiler in general works without vardumper.

The problem is the Request data collector. In RequestDataCollector::lateCollect() the new profiler uses DataCollector::cloneVar() to create a serializable version of the data to be collected. This in turn uses the VarCloner class of the vardumper component if exists or fallback to the ValueExporter class if not. The output of VarCloner can be later used to reconstruct the PHP data structure used to render the Webprofiler's request panel, with ValueExporter this is not possibile.

If we remove the vardumper dependency we need to remove the request data collector also (but it contains a lot of useful information).

That said we can choose to hide the request data panel only for users that doesn't have the vardumper component available (every user that already has Drush installed as Composer dependency has the vardumper component available).