do not constraint check against monomorphs when unifying abstract to-fields (fixed issue 1830)

git-svn-id: http://haxe.googlecode.com/svn/trunk@6660 f16182fa-f095-11de-8f43-4547254af6c6
added List specification

git-svn-id: http://haxe.googlecode.com/svn/trunk@6661 f16182fa-f095-11de-8f43-4547254af6c6