Add Declaration.isAnonymousMixinApplication

Change-Id: Iea0568a6d280db2f811af8579c1146820bbb2c31
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/96652
Reviewed-by: Aske Simon Christensen <askesc@google.com>
diff --git a/pkg/front_end/lib/src/fasta/builder/declaration.dart b/pkg/front_end/lib/src/fasta/builder/declaration.dart
index 1584102..392b3e6 100644
--- a/pkg/front_end/lib/src/fasta/builder/declaration.dart
+++ b/pkg/front_end/lib/src/fasta/builder/declaration.dart
@@ -64,6 +64,10 @@
 
   bool get isNamedMixinApplication => false;
 
+  bool get isAnonymousMixinApplication {
+    return isMixinApplication && !isNamedMixinApplication;
+  }
+
   /// Applies [patch] to this declaration.
   void applyPatch(Declaration patch) {
     unsupported("${runtimeType}.applyPatch", charOffset, fileUri);