Assignments with properties on the left-hand side and compound assignments.

Without doing name resolution, we can't disambiguate a.x where a is an
identifier.  It could be a class name or library prefix, or it could
be a potentially mutable variable.  It is translated as if it were a
mutable variable.
Merge pull request #16 from kmillikin/compound-assignment

Assignments with properties on the left-hand side and compound assignmen...