Don't export unnamed mixins.

pkg/front_end/testcases/rasta/duplicated_mixin.dart.direct.expect

 library;
 import self as self;
 import "dart:core" as core;
 
 class Mixin extends core::Object {
   field dynamic field = null;
   default constructor •() → void
     : super core::Object::•()
     ;
 }
-abstract class Object&Mixin = core::Object with self::Mixin {
+abstract class _Object&Mixin = core::Object with self::Mixin {
 }
-abstract class Object&Mixin&Mixin = self::Object&Mixin with self::Mixin {
+abstract class __Object&Mixin&Mixin = self::_Object&Mixin with self::Mixin {
 }
-class A extends self::Object&Mixin&Mixin {
+class A extends self::__Object&Mixin&Mixin {
   default constructor •() → void
     : super core::Object::•()
     ;
 }