add constants for dart artifacts

[jvasileff]

This PR adds minimal support for Dart file extensions. The need for this is an inability to "get" artifacts (storing isn't a problem!) from a repository manager with unknown suffixes:

com.redhat.ceylon.model.cmr.RepositoryException: Unknown suffix in simple-1.0.0.dart
    at com.redhat.ceylon.cmr.api.ArtifactContext.getSuffixFromFilename_(ArtifactContext.java:291)
    at com.redhat.ceylon.cmr.api.ArtifactContext.getSuffixFromFilename(ArtifactContext.java:259)
    at com.redhat.ceylon.cmr.api.ArtifactContext.getSuffixFromNode(ArtifactContext.java:250)
    at com.redhat.ceylon.cmr.impl.AbstractNodeRepositoryManager.getArtifactResult(AbstractNodeRepositoryManager.java:169)
    at com.redhat.ceylon.cmr.api.AbstractRepositoryManager.getArtifact(AbstractRepositoryManager.java:87)

The Dart backend is currently just an experiment. But it's a big experiment, and potentially useful. Without this patch, it will be much more difficult to let others try it out. So, if it doesn't look like it will cause problems, it would be great if this can be merged.

[quintesse]

Whaddayathink @FroMage ? It's not that this code is unwanted IMO, but it just shows that the current design of the CMR is not meant for easy extension with previously unknown file-types. I wonder if we shouldn't just open it up to any file you want to store/retrieve and only filter what we store on the Herd.

[jvasileff]

Bump.

A more general improvement sounds nice, but does that need to hold up this PR?

[quintesse]

Everybody is on holiday, so it's not a good time to be asking for a quick merge :) I think that people who want to try this out right now can apply a patch.

[jvasileff]

I'm just hoping for a "medium" merge - something for 1.2.

[quintesse]

Sure, no problem, we'll either come up with something better for 1.2 or we'll just merge this temporarily.

[jvasileff]

Great, thanks!

[FroMage]

Good enough for me ;)

[jvasileff]

Thanks!