Fix expected return type in async functions

lib/type_environment.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 kernel.type_environment;
 
 import 'ast.dart';
 import 'class_hierarchy.dart';
 import 'core_types.dart';
 import 'type_algebra.dart';
 
 typedef void ErrorHandler(TreeNode node, String message);
 
 class TypeEnvironment extends SubtypeTester {
   final CoreTypes coreTypes;
   final ClassHierarchy hierarchy;
   InterfaceType thisType;
 
   DartType returnType;
   DartType yieldType;
+  AsyncMarker currentAsyncMarker = AsyncMarker.Sync;
 
   /// An error handler for use in debugging, or `null` if type errors should not
   /// be tolerated.  See [typeError].
   ErrorHandler errorHandler;
 
   TypeEnvironment(this.coreTypes, this.hierarchy);
 
   InterfaceType get objectType => coreTypes.objectClass.rawType;
   InterfaceType get nullType => coreTypes.nullClass.rawType;
   InterfaceType get boolType => coreTypes.boolClass.rawType;
@@ skipping 180 matching lines @@
       var subtypeParameter = subtype.namedParameters[name];
       if (subtypeParameter == null) return false;
       // Termination: Both types shrink in size.
       if (!isSubtypeOf(supertypeParameter, subtypeParameter)) {
         return false;
       }
     }
     return true;
   }
 }