Don't export unnamed mixins.

pkg/front_end/testcases/rasta/issue_000080.dart.strong.expect

Index: pkg/front_end/testcases/rasta/issue_000080.dart.strong.expect
diff --git a/pkg/front_end/testcases/rasta/issue_000080.dart.direct.expect b/pkg/front_end/testcases/rasta/issue_000080.dart.strong.expect
similarity index 70%
copy from pkg/front_end/testcases/rasta/issue_000080.dart.direct.expect
copy to pkg/front_end/testcases/rasta/issue_000080.dart.strong.expect
index 85e3e6af181fc4caad2c36c39c1fc0d0fba7eb4f..5005a6918a94df92775d682f1ce6498f83ad2238 100644
--- a/pkg/front_end/testcases/rasta/issue_000080.dart.direct.expect
+++ b/pkg/front_end/testcases/rasta/issue_000080.dart.strong.expect
@@ -10,9 +10,9 @@ class Mixin extends core::Object {
   method foo() → dynamic
     return 87;
 }
-abstract class Object&Mixin = core::Object with self::Mixin {
+abstract class _Object&Mixin = core::Object with self::Mixin {
 }
-class Foo extends self::Object&Mixin {
+class Foo extends self::_Object&Mixin {
   default constructor •() → void
     : super core::Object::•()
     ;
@@ -22,7 +22,7 @@ class Foo extends self::Object&Mixin {
     return super.{self::Mixin::field};
 }
 static method main() → dynamic {
-  dynamic f = new self::Foo::•();
-  f.field = 42;
-  core::print(f.bar());
+  self::Foo f = new self::Foo::•();
+  f.{self::Mixin::field} = 42;
+  core::print(f.{self::Foo::bar}());
 }