fixes #25482, flatten Futures in strong mode so Future.then works

pkg/analyzer/lib/src/generated/element_handle.dart

 // Copyright (c) 2014, 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 analyzer.src.generated.element_handle;
 
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/dart/element/type.dart';
 import 'package:analyzer/src/dart/element/element.dart';
 import 'package:analyzer/dart/ast/ast.dart';
@@ skipping 736 matching lines @@
   @override
   List<LibraryElement> get importedLibraries => actualElement.importedLibraries;
 
   @override
   List<ImportElement> get imports => actualElement.imports;
 
   @override
   bool get isBrowserApplication => actualElement.isBrowserApplication;
 
   @override
+  bool get isDartAsync => actualElement.isDartAsync;
+
+  @override
   bool get isDartCore => actualElement.isDartCore;
 
   @override
   bool get isInSdk => actualElement.isInSdk;
 
   @override
   ElementKind get kind => ElementKind.LIBRARY;
 
   @override
   FunctionElement get loadLibraryFunction => actualElement.loadLibraryFunction;
@@ skipping 323 matching lines @@
 }
 
 /**
  * TODO(scheglov) invalid implementation
  */
 class WeakReference<T> {
   final T value;
   WeakReference(this.value);
   T get() => value;
 }