Don't export unnamed mixins.

pkg/front_end/testcases/mixin.dart.direct.expect

 library;
 import self as self;
 import "dart:core" as core;
 
-abstract class Object&M1 = core::Object with self::M1 {
+abstract class _Object&M1 = core::Object with self::M1 {
 }
-abstract class Object&M1&M2 = self::Object&M1 with self::M2 {
+abstract class __Object&M1&M2 = self::_Object&M1 with self::M2 {
 }
-class B extends self::Object&M1&M2 {
+class B extends self::__Object&M1&M2 {
   constructor •(dynamic value) → void
     : super core::Object::•()
     ;
 }
 abstract class M1 extends core::Object {
   default constructor •() → void
     : super core::Object::•()
     ;
   method m() → dynamic
     return core::print("M1");
 }
 abstract class M2 extends core::Object {
   default constructor •() → void
     : super core::Object::•()
     ;
   method m() → dynamic
     return core::print("M2");
 }
-class C extends self::Object&M1&M2 {
+class C extends self::__Object&M1&M2 {
   constructor •(dynamic value) → void
     : super core::Object::•()
     ;
 }
 abstract class G1<T extends core::Object> extends core::Object {
   default constructor •() → void
     : super core::Object::•()
     ;
   method m() → dynamic
     return core::print(self::G1::T);
 }
-abstract class Object&G1^^#T0<#T0 extends core::Object> = core::Object with self::G1<self::Object&G1^^#T0::#T0> {
+abstract class _Object&G1^^#T0<#T0 extends core::Object> = core::Object with self::G1<self::_Object&G1^^#T0::#T0> {
 }
-class D<S extends core::Object> extends self::Object&G1^^#T0<self::D::S> {
+class D<S extends core::Object> extends self::_Object&G1^^#T0<self::D::S> {
   default constructor •() → void
     : super core::Object::•()
     ;
 }
 static method main() → dynamic {
   new self::B::•(null).m();
   new self::C::•(null).m();
   new self::D::•<dynamic>().m();
   new self::D::•<core::int>().m();
   new self::D::•<core::List<core::int>>().m();
 }