TreeChecker#transformSym: don't crash on <refinement>

Necessary to get -Ycheck:pickler to pass.

Author: smarter

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

@@ -80,7 +80,8 @@ class TreeChecker extends Phase with SymTransformer {
 
     if (sym.isClass && !sym.isAbsent) {
       val validSuperclass = sym.isPrimitiveValueClass || defn.syntheticCoreClasses.contains(sym) ||
-        (sym eq defn.ObjectClass) || (sym is NoSuperClass) || (sym.asClass.superClass.exists)
+        (sym eq defn.ObjectClass) || (sym is NoSuperClass) || (sym.asClass.superClass.exists) ||
+        sym.isRefinementClass
 
       assert(validSuperclass, i"$sym has no superclass set")
       testDuplicate(sym, seenClasses, "class")