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

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

Index: pkg/analyzer/lib/src/dart/element/element.dart
diff --git a/pkg/analyzer/lib/src/dart/element/element.dart b/pkg/analyzer/lib/src/dart/element/element.dart
index 2e781f78a3a3facc57aed6efb5e11553ff7ba551..e7b6d5ca6103c70a790da6c592ba89bb32b762c8 100644
--- a/pkg/analyzer/lib/src/dart/element/element.dart
+++ b/pkg/analyzer/lib/src/dart/element/element.dart
@@ -2508,17 +2508,17 @@ class FieldFormalParameterElementImpl extends ParameterElementImpl
   FieldElement field;
 
   /**
+   * Initialize a newly created parameter element to have the given [name].
+   */
+  FieldFormalParameterElementImpl(Identifier name) : super.forNode(name);
+
+  /**
    * Initialize a newly created parameter element to have the given [name] and
    * [offset].
    */
   FieldFormalParameterElementImpl.forNameAndOffset(String name, int nameOffset)
       : super(name, nameOffset);
 
-  /**
-   * Initialize a newly created parameter element to have the given [name].
-   */
-  FieldFormalParameterElementImpl(Identifier name) : super.forNode(name);
-
   @override
   bool get isInitializingFormal => true;
 
@@ -3103,6 +3103,9 @@ class LibraryElementImpl extends ElementImpl implements LibraryElement {
       entryPoint != null && isOrImportsBrowserLibrary;
 
   @override
+  bool get isDartAsync => name == "dart.async";
+
+  @override
   bool get isDartCore => name == "dart.core";
 
   @override