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

pkg/front_end/testcases/inference/generic_methods_downwards_inference_fold.dart.strong.expect

Index: pkg/front_end/testcases/inference/generic_methods_downwards_inference_fold.dart.strong.expect
diff --git a/pkg/front_end/testcases/inference/generic_methods_downwards_inference_fold.dart.strong.expect b/pkg/front_end/testcases/inference/generic_methods_downwards_inference_fold.dart.strong.expect
new file mode 100644
index 0000000000000000000000000000000000000000..04f957037916cf45c8fac586c2b51b9ed32d9007
--- /dev/null
+++ b/pkg/front_end/testcases/inference/generic_methods_downwards_inference_fold.dart.strong.expect
@@ -0,0 +1,17 @@
+library test;
+import self as self;
+import "dart:core" as core;
+
+static method test() → void {
+  core::List<core::int> o;
+  core::int y = o.{core::Iterable::fold}<core::int>(0, (core::int x, core::int y) → core::int => x.{core::num::+}(y));
+  dynamic z = o.{core::Iterable::fold}<dynamic>(0, (dynamic x, core::int y) → dynamic => x.+(y));
+  y = z;
+}
+static method functionExpressionInvocation() → void {
+  core::List<core::int> o;
+  core::int y = o.{core::Iterable::fold}.call<core::int>(0, (core::int x, core::int y) → core::int => x.{core::num::+}(y));
+  dynamic z = o.{core::Iterable::fold}.call<dynamic>(0, (dynamic x, core::int y) → dynamic => x.+(y));
+  y = z;
+}
+static method main() → dynamic {}