Introduce JsStrategy skeleton

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 'package:kernel/clone.dart';
 import 'package:kernel/type_algebra.dart';
 
@@ skipping 532 matching lines @@
     _ensureSupertypes(cls, env);
     return env.orderedTypeSet;
   }
 
   int _getHierarchyDepth(KClass cls) {
     _KClassEnv env = _classEnvs[cls.classIndex];
     _ensureSupertypes(cls, env);
     return env.orderedTypeSet.maxDepth;
   }
 
+  ClassEntity _getAppliedMixin(KClass cls) {
+    _KClassEnv env = _classEnvs[cls.classIndex];
+    _ensureSupertypes(cls, env);
+    return env.mixedInType?.element;
+  }
+
   DartType _substByContext(DartType type, InterfaceType context) {
     return type.subst(
         context.typeArguments, _getThisType(context.element).typeArguments);
   }
 
   InterfaceType _getSuperType(KClass cls) {
     _KClassEnv env = _classEnvs[cls.classIndex];
     _ensureSupertypes(cls, env);
     return env.supertype;
   }
@@ skipping 898 matching lines @@
     return false;
   }
 
   @override
   int getHierarchyDepth(ClassEntity cls) {
     return elementMap._getHierarchyDepth(cls);
   }
 
   @override
   ClassEntity getAppliedMixin(ClassEntity cls) {
-    // TODO(johnniwinther): Implement this.
-    return null;
+    return elementMap._getAppliedMixin(cls);
   }
 
   @override
   bool validateClass(ClassEntity cls) => true;
 
   @override
   bool checkClass(ClassEntity cls) => true;
 }
 
 // Interface for testing equivalence of Kernel-based entities.
@@ skipping 65 matching lines @@
     return node.isExternal &&
         !elementMap.isForeignLibrary(node.enclosingLibrary);
   }
 
   @override
   bool isJsInteropMember(MemberEntity element) {
     // TODO(johnniwinther): Compute this.
     return false;
   }
 }