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

pkg/analyzer/lib/dart/element/element.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.dart.element.element;
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/element/type.dart';
 import 'package:analyzer/src/generated/constant.dart' show DartObject;
 import 'package:analyzer/src/generated/engine.dart' show AnalysisContext;
@@ skipping 1334 matching lines @@
    * browser.
    */
   bool get isBrowserApplication;
 
   /**
    * Return `true` if this library is the dart:core library.
    */
   bool get isDartCore;
 
   /**
+   * Return `true` if this library is the dart:async library.
+   */
+  bool get isDartAsync;
+
+  /**
    * Return `true` if this library is part of the SDK.
    */
   bool get isInSdk;
 
   /**
    * Return the element representing the synthetic function `loadLibrary` that
    * is implicitly defined for this library if the library is imported using a
    * deferred import.
    */
   FunctionElement get loadLibraryFunction;
@@ skipping 679 matching lines @@
    */
   bool get isStatic;
 
   /**
    * Return the declared type of this variable, or `null` if the variable did
    * not have a declared type (such as if it was declared using the keyword
    * 'var').
    */
   DartType get type;
 }