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

pkg/analyzer/lib/dart/element/element.dart

Index: pkg/analyzer/lib/dart/element/element.dart
diff --git a/pkg/analyzer/lib/dart/element/element.dart b/pkg/analyzer/lib/dart/element/element.dart
index 01e757d8e73181471592605f03731f619ca540d4..b285ca6c35b9da81f6535b6770897422d97a4321 100644
--- a/pkg/analyzer/lib/dart/element/element.dart
+++ b/pkg/analyzer/lib/dart/element/element.dart
@@ -1352,6 +1352,11 @@ abstract class LibraryElement implements Element {
   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;