handle OpAssignOp for abstract getter + setter combination (fixed issue #1754)

git-svn-id: http://haxe.googlecode.com/svn/trunk@6503 f16182fa-f095-11de-8f43-4547254af6c6
allow access to fields of underlying types only on "this"

git-svn-id: http://haxe.googlecode.com/svn/trunk@6504 f16182fa-f095-11de-8f43-4547254af6c6