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

pkg/front_end/testcases/inference/generic_methods_downwards_inference_fold.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;
 
-void main() {
+void test() {
   List<int> o;
-  int y = o. /*@typeArgs=int*/ /*@target=List::fold*/ fold(
+  int y = o. /*@typeArgs=int*/ /*@target=Iterable::fold*/ fold(
       0,
       /*@returnType=int*/ (/*@type=int*/ x,
               /*@type=int*/ y) =>
           x /*@target=num::+*/ + y);
   var /*@type=dynamic*/ z =
-      o. /*@typeArgs=dynamic*/ /*@target=List::fold*/ fold(
+      o. /*@typeArgs=dynamic*/ /*@target=Iterable::fold*/ fold(
           0,
           /*@returnType=dynamic*/ (/*@type=dynamic*/ x,
               /*@type=int*/ y) => /*info:DYNAMIC_INVOKE*/ x + y);
   y = /*info:DYNAMIC_CAST*/ z;
 }
 
 void functionExpressionInvocation() {
   List<int> o;
-  int y = (o. /*@target=List::fold*/ fold) /*@typeArgs=int*/ (
+  int y = (o. /*@target=Iterable::fold*/ fold) /*@typeArgs=int*/ (
       0,
       /*@returnType=int*/ (/*@type=int*/ x,
               /*@type=int*/ y) =>
           x /*@target=num::+*/ + y);
   var /*@type=dynamic*/ z =
-      (o. /*@target=List::fold*/ fold) /*@typeArgs=dynamic*/ (
+      (o. /*@target=Iterable::fold*/ fold) /*@typeArgs=dynamic*/ (
           0,
           /*@returnType=dynamic*/ (/*@type=dynamic*/ x,
               /*@type=int*/ y) => /*info:DYNAMIC_INVOKE*/ x + y);
   y = /*info:DYNAMIC_CAST*/ z;
 }
+
+main() {}