Implement unresolved super-send (but ignoring arguments).

dart/frog/leg/elements/elements.dart

Index: dart/frog/leg/elements/elements.dart
diff --git a/dart/frog/leg/elements/elements.dart b/dart/frog/leg/elements/elements.dart
index b226a42bd73afdd5e369b0b7eb494649999d7bc3..3d17f19db5d40200fcb5a146d01313ff9488fd9d 100644
--- a/dart/frog/leg/elements/elements.dart
+++ b/dart/frog/leg/elements/elements.dart
@@ -171,6 +171,13 @@ class Element implements Hashable {
     return element;
   }
 
+  ClassElement getEnclosingClass() {
+    for (Element e = this; e !== null; e = e.enclosingElement) {
+      if (e.kind === ElementKind.CLASS) return e;
+    }
+    return null;
+  }
+
   toString() => '$kind(${name.slowToString()})';
 
   bool _isNative = false;