Anyway, because I couldn't find a way (or let's say there is nothing about it on web) to get aspect definitions from alfresco via webscripts I decided to use web services.
I'm getting ClassDefinition[] from alfresco using DictionaryServiceSoapBindingStub, but I couldn't access mandatory aspects defined for an aspect. How can I get mandatory aspects of an aspect?
I don't think that you can get mandatory-aspect definitions using webservice DictionaryService and I'm not aware of any way to do this via Alfresco webservices.