The plugin code in javac.main.Main should be invoked from JavacTaskImpl.prepareCompiler

Currently, plugin support (and upcoming doclint support) are only invoked during a full compilation, either from the command line or via the JSR 199 call/run methods. They are not run if the user downcasts CompilationTask to a JavacTask and calls parse/analyze/generate directly.

This is not a huge deal since anyone running the API at that level has access to registering task listeners directly, but it would be nice to provide uniformity.