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

pkg/front_end/testcases/inference/infer_assign_to_index_super_upwards.dart.strong.expect

Index: pkg/front_end/testcases/inference/infer_assign_to_index_super_upwards.dart.strong.expect
diff --git a/pkg/front_end/testcases/inference/infer_assign_to_index_super_upwards.dart.strong.expect b/pkg/front_end/testcases/inference/infer_assign_to_index_super_upwards.dart.strong.expect
index 0b50ad1da92aaf465298048f83b5e01e8a680f18..d6c2a82d0c3eeea42cd6eaf6e66a8d35be40fe70 100644
--- a/pkg/front_end/testcases/inference/infer_assign_to_index_super_upwards.dart.strong.expect
+++ b/pkg/front_end/testcases/inference/infer_assign_to_index_super_upwards.dart.strong.expect
@@ -6,8 +6,12 @@ abstract class Base<T extends core::Object, U extends core::Object> extends core
   constructor •() → void
     : super core::Object::•()
     ;
-  abstract operator [](core::String s) → self::Base::T;
-  abstract operator []=(core::String s, self::Base::U v) → void;
+  operator [](core::String s) → self::Base::T
+    return this.{self::Base::getValue}(s);
+  operator []=(core::String s, self::Base::U v) → void
+    return this.{self::Base::setValue}(s, v);
+  abstract method getValue(core::String s) → self::Base::T;
+  abstract method setValue(core::String s, self::Base::U v) → void;
 }
 abstract class Test1 extends self::Base<core::int, core::int> {
   constructor •() → void