Move ElementEnvironment.isSubtype to DartTypes

pkg/compiler/lib/src/kernel/kernel_strategy.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.frontend_strategy;
 
 import '../closure.dart';
 import '../backend_strategy.dart';
 import '../common.dart';
 import '../common_elements.dart';
@@ skipping 48 matching lines @@
       env.Environment environment,
       DiagnosticReporter reporter,
       Measurer measurer) {
     return new DillLibraryLoaderTask(
         elementMap, uriTranslator, scriptLoader, reporter, measurer);
   }
 
   @override
   ElementEnvironment get elementEnvironment => elementMap.elementEnvironment;
 
+  DartTypes get dartTypes => elementMap.types;
+
   @override
   AnnotationProcessor get annotationProcesser =>
       _annotationProcesser ??= new KernelAnnotationProcessor(elementMap);
 
   @override
   NativeClassFinder createNativeClassFinder(NativeBasicData nativeBasicData) {
     return new BaseNativeClassFinder(elementMap.elementEnvironment,
         elementMap.commonElements, nativeBasicData);
   }
 
@@ skipping 145 matching lines @@
   Sorter get sorter =>
       throw new UnimplementedError('KernelBackendStrategy.sorter');
 
   @override
   void convertClosures(ClosedWorldRefiner closedWorldRefiner) {
     // TODO(johnniwinther,efortuna): Compute closure classes for kernel based
     // elements.
     throw new UnimplementedError('KernelBackendStrategy.createClosureClasses');
   }
 }