Select operations in erasure should fix symbols in types.

Otherwise we get binding races, e.g. for pos/t2133.scala after mixin
because we create same-named symbols in subclasses, and type refs now
can refer to the subclass symbol where previously they refereed to the
superclass symbol. To avoid a data race the new term ref will have a
fixed symbol.

Author: odersky

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

@@ -277,7 +277,7 @@ object Erasure extends TypeTestsCasts{
           case _ => sym.name
         }
         untpd.cpy.Select(tree)(qual, sym.name)
-          .withType(NamedType.withSymAndName(qual.tpe, sym, name))
+          .withType(NamedType.withFixedSym(qual.tpe, sym))
       }
 
       def selectArrayMember(qual: Tree, erasedPre: Type): Tree =