Map of style names to metadata declarations
Note: for now anyway, we simply point to the original metadata. Higher-level wrappers are implemented by each compiler's reflection package. But clearly, if we throw exceptions on inequivalent adds (or generally do anything more than a straight, low-level equality test when testing), then there's higher-level awareness here, and we should factor out something like mxml.reflect.TypeTable.StyleHelper to here.
Note: both old [Style] and new [StyleProperty] metadata formats may be stored.
@author Paul Reilly
@author Pete Farland