Fix implementation of fold() in analyzer's mock SDK.

pkg/front_end/testcases/inference/generic_methods_iterable_and_future.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;
 
 import 'dart:async';
 
 Future<int> make(int x) => (new /*@typeArgs=int*/ Future(
     /*@returnType=int*/ () => x));
 
 main() {
   Iterable<Future<int>> list = <int>[1, 2, 3]
       . /*@typeArgs=Future<int>*/ /*@target=Iterable::map*/ map(make);
   Future<List<int>> results = Future. /*@typeArgs=int*/ wait(list);
   Future<String> results2 =
       results. /*@typeArgs=String*/ /*@target=Future::then*/ then(
-          /*@returnType=FutureOr<String>*/ (List<int> list) =>
-              list. /*@typeArgs=FutureOr<String>*/ /*@target=List::fold*/ fold(
+          /*@returnType=FutureOr<String>*/ (List<int> list) => list
+              . /*@typeArgs=FutureOr<String>*/ /*@target=Iterable::fold*/ fold(
                   '',
                   /*@returnType=FutureOr<String>*/ (/*@type=FutureOr<String>*/ x,
                           /*@type=int*/ y) => /*info:DYNAMIC_CAST,info:DYNAMIC_INVOKE*/ x /*error:UNDEFINED_OPERATOR*/ +
                       y. /*@target=Object::toString*/ toString()));
 
   Future<String> results3 =
       results. /*@typeArgs=String*/ /*@target=Future::then*/ then(
-          /*@returnType=FutureOr<String>*/ (List<int> list) =>
-              list. /*@typeArgs=FutureOr<String>*/ /*@target=List::fold*/ fold(
+          /*@returnType=FutureOr<String>*/ (List<int> list) => list
+              . /*@typeArgs=FutureOr<String>*/ /*@target=Iterable::fold*/ fold(
                   '',
                   /*info:INFERRED_TYPE_CLOSURE,error:ARGUMENT_TYPE_NOT_ASSIGNABLE*/ /*@returnType=String*/ (String
                               x,
                           /*@type=int*/ y) =>
                       x /*@target=String::+*/ +
                       y. /*@target=Object::toString*/ toString()));
 
   Future<String> results4 =
       results. /*@typeArgs=String*/ /*@target=Future::then*/ then(
           /*@returnType=String*/ (List<int> list) =>
-              list. /*@target=List::fold*/ fold<String>(
+              list. /*@target=Iterable::fold*/ fold<String>(
                   '',
                   /*@returnType=String*/ (/*@type=String*/ x,
                           /*@type=int*/ y) =>
                       x /*@target=String::+*/ +
                       y. /*@target=Object::toString*/ toString()));
 }