Capability: Access metadata about reflectable.dart

[eernstg]

Please see #6 on dartbug 15704 for some additional information. Reflectable mirrors are intended to be very similar to mirrors from dart:mirrors, so they should agree on the treatment of doc comments.

However, this feature can be implemented independently in dart:mirrors and in Reflectable for post-transform code (whereas pre-transform code in Reflectable relies on dart:mirrors), so I added a reminder in the code to include comments in metadata of DeclarationMirror.

Currently dart:mirrors mirrors do not include doc (or other) comment mirrors in the metadata, but when they do that, there may be a need to adjust the semantics for precise alignment among dart:mirrors and Reflectable. (As an extreme example of this, the outcome may even be that it is decided that dart:mirrors should not support comments as metadata, for backward compatibility or whatever, in which case Reflectable may have to align with the lack of support). In any case, there will be a "temporary feature, do not rely on this in production code" flag on this feature for now.

Also note that the implementation of Reflectable is still highly incomplete, so it will not yet be possible to try it out at this point.