Work around for private names in mixin applications.

test/kernel/regression/unresolved_constructor.dart.txt

 library;
 import self as self;
 import "dart:core" as core;
 
 class Foo extends core::Object {
   constructor •(dynamic x, dynamic y) → dynamic
     : super core::Object::•()
     ;
 }
 static method main() → dynamic {
   invalid-expression;
-  throw core::_unresolvedConstructorError(self::Foo, #Foo.notHere, <dynamic>[], <dynamic, dynamic>{}, null);
+  throw core::dart.core::_unresolvedConstructorError(self::Foo, #Foo.notHere, <dynamic>[], <dynamic, dynamic>{}, null);
 }