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

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

 library test;
 import self as self;
 import "dart:core" as core;
 
 abstract class Base<T extends core::Object, U extends core::Object> extends core::Object {
   constructor •() → void
     ;
-  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
+    ;
+  operator []=(core::String s, self::Base::U v) → void
+    ;
+  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
     ;
   method test() → void
     ;
 }
 abstract class Test2 extends self::Base<core::int, core::num> {
   constructor •() → void
     ;
@@ skipping 43 matching lines @@
     ;
 }
 static method getInt() → core::int
   ;
 static method getNum() → core::num
   ;
 static method getDouble() → core::double
   ;
 static method main() → dynamic
   ;