Bug 940: Access to FileSystemEnvironment

From http://code.google.com/p/google-caja/issues/detail?id=940

    While the class FileSystemEnvironment is public, its only
    constructor isn't, which means there's currently no way to derive
    from it outside of the caja.plugin package. Is this by design?
    It's a useful base class and it'd be nice to be able to take
    advantage of it.

    Similarly, it'd be pretty useful if PluginCompilerMain's private
    CachingEnvironment were factored out into an independent public
    class.


Submitted @3758

[metaweta, 2009-09-26 17:21:22]

LGTM