There were three classes in Solr 3.6 for creating custom analyzers. Those classes were the BaseCharFilterFactory, BaseTokenizerFactory, and the BaseTokenFilterFactory. These three classes were in the documented package, org.apache.solr.analysis, through Solr 4.0 ALPHA, but as of the BETA release, they have been moved and renamed.
In Solr 4.0, the new classes are the CharFilterFactory, TokenizerFactory, and TokenFilterFactory. They can be found in the org.apache.lucene.analysis.util package, which is part of Lucene's analyzers-common project