Fix returns inside while cycles.

typing of Returns looked for directly enclosing method,
but didn't take in account presence of synthesised labels.
So if one had a return inside a while loop it would be
adapted to type of while, which is always unit.
The adaption always succeeds and it was left unnoticed.

Author: DarkDimius

src/dotty/tools/dotc/typer/Typer.scala

@@ -677,7 +677,7 @@ class Typer extends Namer with TypeAssigner with Applications with Implicits wit
         if (cx == NoContext || owner.isType) {
           ctx.error("return outside method definition", tree.pos)
           (EmptyTree, WildcardType)
-        } else if (owner.isSourceMethod)
+        } else if (owner.isSourceMethod && !(owner is Flags.Label))
           if (owner.isCompleted) {
             val from = Ident(TermRef(NoPrefix, owner.asTerm))
             val proto = returnProto(owner)