Make checkCompanion stable against future Defs

Author: DarkDimius

src/dotty/tools/dotc/transform/TreeChecker.scala

@@ -59,8 +59,10 @@ class TreeChecker extends Phase with SymTransformer {
   def checkCompanion(symd: SymDenotation)(implicit ctx: Context): Unit = {
     val cur = symd.linkedClass
     val prev = ctx.atPhase(ctx.phase.prev) {
-      implicit  ctx =>
+      ct => {
+        implicit val ctx: Context = ct.withMode(Mode.FutureDefsOK)
         symd.symbol.linkedClass
+      }
     }
 
     if (prev.exists)