Move ElementEnvironment.isSubtype to DartTypes

pkg/compiler/lib/src/kernel/element_map_impl.dart

 // Copyright (c) 2017, 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.element_map;
 
 import 'package:kernel/ast.dart' as ir;
 
 import '../common.dart';
 import '../common/names.dart' show Identifiers;
@@ skipping 932 matching lines @@
         'KernelElementEnvironment.getTypeVariableBound');
   }
 
   @override
   InterfaceType createInterfaceType(
       ClassEntity cls, List<DartType> typeArguments) {
     return new InterfaceType(cls, typeArguments);
   }
 
   @override
-  bool isSubtype(DartType a, DartType b) {
-    return elementMap.types.isSubtype(a, b);
-  }
-
-  @override
   FunctionType getFunctionType(KFunction function) {
     return elementMap._getFunctionType(function);
   }
 
   @override
   FunctionType getLocalFunctionType(KLocalFunction function) {
     return function.functionType;
   }
 
   @override
@@ skipping 345 matching lines @@
   }
 
   InterfaceType getMixinTypeForClass(KClass cls) {
     _KClassEnv env = elementMap._classEnvs[cls.classIndex];
     ir.Supertype mixedInType = env.cls.mixedInType;
     if (mixedInType == null) return null;
     return elementMap.createInterfaceType(
         mixedInType.classNode, mixedInType.typeArguments);
   }
 }