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

pkg/front_end/testcases/inference/infer_assign_to_index_super_upwards.dart

 // Copyright (c) 2017, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 /*@testedFeatures=inference*/
 library test;
 
 int getInt() => 0;
 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> {
   void test() {
     var /*@type=int*/ v1 = super /*@target=Base::[]=*/ ['x'] = getInt();
     var /*@type=num*/ v2 = super /*@target=Base::[]=*/ ['x'] = getNum();
     var /*@type=int*/ v4 = super /*@target=Base::[]=*/ ['x'] ??= getInt();
     var /*@type=num*/ v5 = super /*@target=Base::[]=*/ ['x'] ??= getNum();
     var /*@type=int*/ v7 = super /*@target=Base::[]=*/ ['x'] += getInt();
     var /*@type=num*/ v8 = super /*@target=Base::[]=*/ ['x'] += getNum();
@@ skipping 112 matching lines @@
     var /*@type=double*/ v6 = super /*@target=Base::[]=*/ ['x'] ??= getDouble();
     var /*@type=double*/ v7 = super /*@target=Base::[]=*/ ['x'] += getInt();
     var /*@type=double*/ v8 = super /*@target=Base::[]=*/ ['x'] += getNum();
     var /*@type=double*/ v9 = super /*@target=Base::[]=*/ ['x'] += getDouble();
     var /*@type=double*/ v10 = ++super /*@target=Base::[]=*/ ['x'];
     var /*@type=double*/ v11 = super /*@target=Base::[]=*/ ['x']++;
   }
 }
 
 main() {}