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

 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
     : 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
     : super self::Base::•()
     ;
   method test() → void {
     core::int v1 = let final dynamic #t1 = "x" in let final dynamic #t2 = self::getInt() in let final dynamic #t3 = super.{self::Base::[]=}(#t1, #t2) in #t2;
     core::num v2 = let final dynamic #t4 = "x" in let final dynamic #t5 = self::getNum() in let final dynamic #t6 = super.{self::Base::[]=}(#t4, #t5) in #t5;
     core::int v4 = let final dynamic #t7 = "x" in let final dynamic #t8 = super.{self::Base::[]}(#t7) in #t8.{core::num::==}(null) ? let final dynamic #t9 = self::getInt() in let final dynamic #t10 = super.{self::Base::[]=}(#t7, #t9) in #t9 : #t8;
     core::num v5 = let final dynamic #t11 = "x" in let final dynamic #t12 = super.{self::Base::[]}(#t11) in #t12.{core::num::==}(null) ? let final dynamic #t13 = self::getNum() in let final dynamic #t14 = super.{self::Base::[]=}(#t11, #t13) in #t13 : #t12;
@@ skipping 135 matching lines @@
     core::double v11 = let final dynamic #t271 = "x" in let final dynamic #t272 = super.{self::Base::[]}(#t271) in let final dynamic #t273 = super.{self::Base::[]=}(#t271, #t272.{core::double::+}(1)) in #t272;
   }
 }
 static method getInt() → core::int
   return 0;
 static method getNum() → core::num
   return 0;
 static method getDouble() → core::double
   return 0.0;
 static method main() → dynamic {}