Fix fasta testcase to avoid a super call to an abstract method.

pkg/front_end/testcases/inference/infer_assign_to_index_super_upwards.dart

Index: pkg/front_end/testcases/inference/infer_assign_to_index_super_upwards.dart
diff --git a/pkg/front_end/testcases/inference/infer_assign_to_index_super_upwards.dart b/pkg/front_end/testcases/inference/infer_assign_to_index_super_upwards.dart
index 8b765d651dc535cb38961add7141fc031a726763..a2d7ab6f8708270c30d174c3cf3f4259f2294483 100644
--- a/pkg/front_end/testcases/inference/infer_assign_to_index_super_upwards.dart
+++ b/pkg/front_end/testcases/inference/infer_assign_to_index_super_upwards.dart
@@ -10,8 +10,11 @@ num getNum() => 0;
 double getDouble() => 0.0;
 
 abstract class Base<T, U> {
-  T operator [](String s);
-  void operator []=(String s, U v);
+  T operator [](String s) => /*@target=Base::getValue*/ getValue(s);
+  void operator []=(String s, U v) => /*@target=Base::setValue*/ setValue(s, v);
+
+  T getValue(String s);
+  void setValue(String s, U v);
 }
 
 abstract class Test1 extends Base<int, int> {