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

pkg/front_end/testcases/inference/generic_methods_iterable_and_future.dart

Index: pkg/front_end/testcases/inference/generic_methods_iterable_and_future.dart
diff --git a/pkg/front_end/testcases/inference/generic_methods_iterable_and_future.dart b/pkg/front_end/testcases/inference/generic_methods_iterable_and_future.dart
index b21b7e40466575af1f681641f185656ae882b553..0de66c816a1995c024b08cc113644c11c9ea2698 100644
--- a/pkg/front_end/testcases/inference/generic_methods_iterable_and_future.dart
+++ b/pkg/front_end/testcases/inference/generic_methods_iterable_and_future.dart
@@ -16,8 +16,8 @@ main() {
   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*/ +
@@ -25,8 +25,8 @@ main() {
 
   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,
@@ -37,7 +37,7 @@ main() {
   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) =>