fix #28008, fix #28009 implement FutureOr<T>

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

Index: pkg/analyzer/lib/dart/element/type.dart
diff --git a/pkg/analyzer/lib/dart/element/type.dart b/pkg/analyzer/lib/dart/element/type.dart
index fe0776559211f1859361c85caf551b82a1ac68ef..4f0729c7c63454b5a9609c61b015dcb25d33c362 100644
--- a/pkg/analyzer/lib/dart/element/type.dart
+++ b/pkg/analyzer/lib/dart/element/type.dart
@@ -64,6 +64,12 @@ abstract class DartType implements ResolutionType {
   bool get isDartAsyncFuture;
 
   /**
+   * Return `true` if this type represents the type 'FutureOr<T>' defined in the
+   * dart:async library.
+   */
+  bool get isDartAsyncFutureOr;
+
+  /**
    * Return `true` if this type represents the type 'Function' defined in the
    * dart:core library.
    */