[phobos] phobos commit, revision 1683

[Walter Bright]

This breaks the unit tests:

Error: AssertError Failure std\utf.d(433)

dsource.org wrote:
> phobos commit, revision 1683
>
>
> user: rsinfu
>
> msg:
> [1.x] Fixed bugzilla 978: std.utf's toUTF* functions accept some invalid and reject some valid UTF.
>
> * Fixed decode() to accept U+FFFE and U+FFFF.
> * Changed some assert contracts (which check input for validity) to if-throw.
>
> http://www.dsource.org/projects/phobos/changeset/1683
>
>