Handle FutureOr type when inferring async closures.

pkg/kernel/lib/core_types.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.core_types;
 
 import 'ast.dart';
 import 'library_index.dart';
 
 /// Provides access to the classes and libraries in the core libraries.
 class CoreTypes extends LibraryIndex {
   Class objectClass;
   Class nullClass;
   Class boolClass;
   Class intClass;
   Class numClass;
   Class doubleClass;
   Class stringClass;
   Class listClass;
   Class mapClass;
   Class iterableClass;
   Class iteratorClass;
   Class futureClass;
+  Class futureOrClass;
   Class streamClass;
   Class symbolClass;
   Class internalSymbolClass;
   Class typeClass;
   Class functionClass;
   Class invocationClass;
 
   static final Map<String, List<String>> requiredClasses = {
     'dart:core': [
       'Object',
@@ skipping 31 matching lines @@
     stringClass = getClass('dart:core', 'String');
     listClass = getClass('dart:core', 'List');
     mapClass = getClass('dart:core', 'Map');
     iterableClass = getClass('dart:core', 'Iterable');
     iteratorClass = getClass('dart:core', 'Iterator');
     symbolClass = getClass('dart:core', 'Symbol');
     typeClass = getClass('dart:core', 'Type');
     functionClass = getClass('dart:core', 'Function');
     invocationClass = getClass('dart:core', 'Invocation');
     futureClass = getClass('dart:async', 'Future');
+    futureOrClass = getClass('dart:async', 'FutureOr');
     streamClass = getClass('dart:async', 'Stream');
     internalSymbolClass = getClass('dart:_internal', 'Symbol');
   }
 }