Tighten isStable test.

A type like C#x is not stable even if x is an
immutable value, because the prefix C is not a
singleton. This is now taken into account.

Author: odersky

src/dotty/tools/dotc/core/Types.scala

@@ -92,7 +92,7 @@ object Types {
 
     /** Does this type denote a stable reference (i.e. singleton type)? */
     final def isStable(implicit ctx: Context): Boolean = this match {
-      case tp: TermRef => tp.termSymbol.isStable
+      case tp: TermRef => tp.termSymbol.isStable && tp.prefix.isStable
       case _: SingletonType => true
       case NoPrefix => true
       case _ => false