Add more info on missing references in unpickler.

Trying to hunt down the flakey build.

Author: odersky

src/dotty/tools/dotc/core/unpickleScala2/Scala2Unpickler.scala

@@ -415,6 +415,8 @@ class Scala2Unpickler(bytes: Array[Byte], classRoot: ClassDenotation, moduleClas
               // println(owner.info.decls.toList.map(_.debugString).mkString("\n  ")) // !!! DEBUG
               //              }
               // (5) Create a stub symbol to defer hard failure a little longer.
+              println(i"***** missing reference, looking for $name in $owner")
+              println(i"decls = ${owner.info.decls}")
               new Exception().printStackTrace()
               ctx.newStubSymbol(owner, name, source)
             }