Add a new special case to disjointness analysis
fix npe
oh, the "special case" #1138 can actually be safely generalized!
oops, did not mean to commit that
Unit: made method protected so the compiler can use it in ceylon/ceylon-compiler#1908
fix for fix for #1136
Merge branch 'master' into constructors

Conflicts:
	src/com/redhat/ceylon/compiler/typechecker/model/Util.java