Fix crashes

Unlike in Parsers, at this point in Desugar we get Synthetic objects.

We also get objects with flags but without corresponding mods; they might all
be synthetic, but let's harden flagPos anyway.

Author: Blaisorblade

compiler/src/dotty/tools/dotc/ast/Desugar.scala

@@ -701,14 +701,14 @@ object desugar {
     val mods = mdef.mods
     def isEnumCase = mods.isEnumCase
 
-    def flagPos(flag: FlagSet) = mods.mods.find(_.flags == flag).get.pos
+    def flagPos(flag: FlagSet) = mods.mods.find(_.flags == flag).fold(mdef.pos)(_.pos)
 
     if (mods.is(Abstract))
       ctx.error(hl"""$Abstract modifier cannot be used for objects""", flagPos(Abstract))
     if (mods.is(Sealed))
       ctx.error(hl"""$Sealed modifier is redundant for objects""", flagPos(Sealed))
     // Maybe this should be an error; see https://github.com/scala/bug/issues/11094.
-    if (mods.is(Final))
+    if (mods.is(Final) && !mods.is(Synthetic))
       ctx.warning(hl"""$Final modifier is redundant for objects""", flagPos(Final))
 
     if (mods is Package)