Implement subtype relation for kernel based elements.

pkg/compiler/lib/src/constant_system_dart.dart

 // Copyright (c) 2012, 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.constant_system.dart;
 
 import 'constants/constant_system.dart';
 import 'constants/values.dart';
 import 'common_elements.dart' show CommonElements;
 import 'elements/types.dart';
-import 'elements/resolution_types.dart' show DartTypes;
 import 'tree/dartstring.dart' show DartString;
 
 const DART_CONSTANT_SYSTEM = const DartConstantSystem();
 
 class BitNotOperation implements UnaryOperation {
   final String name = '~';
   const BitNotOperation();
   ConstantValue fold(ConstantValue constant) {
     if (constant.isInt) {
       IntConstantValue intConstant = constant;
@@ skipping 445 matching lines @@
   bool isInt(ConstantValue constant) => constant.isInt;
   bool isDouble(ConstantValue constant) => constant.isDouble;
   bool isString(ConstantValue constant) => constant.isString;
   bool isBool(ConstantValue constant) => constant.isBool;
   bool isNull(ConstantValue constant) => constant.isNull;
 
   bool isSubtype(DartTypes types, DartType s, DartType t) {
     return types.isSubtype(s, t);
   }
 }