consider using-fields only if left type has no monomorphs, and then also check constraints (fixed issue #1466)

git-svn-id: http://haxe.googlecode.com/svn/trunk@6181 f16182fa-f095-11de-8f43-4547254af6c6
added DateTools.makeUtc

git-svn-id: http://haxe.googlecode.com/svn/trunk@6182 f16182fa-f095-11de-8f43-4547254af6c6
Merge branch 'haxe/master'