Don't export unnamed mixins.

pkg/front_end/testcases/inference/infer_field_overrides_getter.dart.direct.expect

 library test;
 import self as self;
 import "dart:core" as core;
 
 abstract class A extends core::Object {
   default constructor •() → void
     : super core::Object::•()
     ;
   abstract get x() → core::int;
 }
@@ skipping 21 matching lines @@
   default constructor •() → void
     : super core::Object::•()
     ;
 }
 class F extends core::Object implements self::B {
   field dynamic x = null;
   default constructor •() → void
     : super core::Object::•()
     ;
 }
-abstract class Object&B = core::Object with self::B {
+abstract class _Object&B = core::Object with self::B {
 }
-class G extends self::Object&B {
+class G extends self::_Object&B {
   field dynamic x = null;
   default constructor •() → void
     : super core::Object::•()
     ;
 }
 static method main() → dynamic {}