Extract DartTypeVisitor and subclasses from resolution_types.

tests/compiler/dart2js/kernel/impact_test.dart

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library dart2js.kernel.impact_test;
 
 import 'package:async_helper/async_helper.dart';
 import 'package:compiler/src/commandline_options.dart';
 import 'package:compiler/src/common.dart';
 import 'package:compiler/src/common/names.dart';
@@ skipping 840 matching lines @@
         builder.registerFeature(feature);
         break;
     }
   }
   impact.nativeData.forEach(builder.registerNativeData);
   return builder;
 }
 
 /// Visitor the performers unaliasing of all typedefs nested within a
 /// [ResolutionDartType].
-class Unaliaser extends BaseDartTypeVisitor<dynamic, ResolutionDartType> {
+class Unaliaser
+    extends BaseResolutionDartTypeVisitor<dynamic, ResolutionDartType> {
   const Unaliaser();
 
   @override
   ResolutionDartType visit(ResolutionDartType type, [_]) =>
       type.accept(this, null);
 
   @override
   ResolutionDartType visitType(ResolutionDartType type, _) => type;
 
   List<ResolutionDartType> visitList(List<ResolutionDartType> types) =>
@@ skipping 17 matching lines @@
         visitList(type.optionalParameterTypes),
         type.namedParameters,
         visitList(type.namedParameterTypes));
   }
 }
 
 /// Perform unaliasing of all typedefs nested within a [ResolutionDartType].
 ResolutionDartType unalias(ResolutionDartType type) {
   return const Unaliaser().visit(type);
 }