Fix for #371

This causes backend to fail with ambiguous reference when loading
member "equals" of object "BoxesRuntime".

The reference is indeed ambiguous. BoxesRunTime has a two paremeter
equals method and, with the fix, inherits the one-parameter method from
Object. The backend needs to disambiguate, e.g. by demanding the `decl`
equals in BoxesRunTime instead of the member.

Author: odersky

src/dotty/tools/dotc/core/pickling/UnPickler.scala

@@ -106,7 +106,8 @@ object UnPickler {
       case TempPolyType(tps, cinfo) => (tps, cinfo)
       case cinfo => (Nil, cinfo)
     }
-    val parentRefs = ctx.normalizeToClassRefs(parents, cls, decls)
+    var parentRefs = ctx.normalizeToClassRefs(parents, cls, decls)
+    if (parentRefs.isEmpty) parentRefs = defn.ObjectClass.typeRef :: Nil
     for (tparam <- tparams) {
       val tsym = decls.lookup(tparam.name)
       if (tsym.exists) tsym.setFlag(TypeParam)