Add LibraryElementForLink.isDartAsync.

pkg/analyzer/lib/src/summary/link.dart

Index: pkg/analyzer/lib/src/summary/link.dart
diff --git a/pkg/analyzer/lib/src/summary/link.dart b/pkg/analyzer/lib/src/summary/link.dart
index 9e9d19e838b779772cd74d2d4c1c84a8723cbd4d..a4b896dd97198bba49ec40695821f1fd7ed2a6e6 100644
--- a/pkg/analyzer/lib/src/summary/link.dart
+++ b/pkg/analyzer/lib/src/summary/link.dart
@@ -2705,6 +2705,9 @@ abstract class LibraryElementForLink<
   List<LibraryElementForLink> get importedLibraries => _importedLibraries ??=
       _linkedLibrary.importDependencies.map(_getDependency).toList();
 
+  @override
+  bool get isDartAsync => _absoluteUri == 'dart:async';
+
   /**
    * If this library is part of the build unit being linked, return the library
    * cycle it is part of.  Otherwise return `null`.