It is fairly clear that Assange is not covered by Refugee Convention and is therefore not entitled to asylum as a matter of international law. (blog.politics.ox.ac.uk)
In contrast to treaties, which require two - thirds of the Senate to consent to ratification, political commitments require no congressional approval, and are not legally binding as a matter of domestic law (although in some cases they may be binding on the U.S. as a matter of international law). (politics.stackexchange.com)
In a previous post, my colleague Stefan Talmon explained that declarations that the Gaddafi regime is illegitimate does not mean that that regime is not (and is not considered to be) the government of Libya as a matter of international law (and in the domestic law of other countries). (blog.politics.ox.ac.uk)