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

pkg/analyzer/test/src/context/mock_sdk.dart

Index: pkg/analyzer/test/src/context/mock_sdk.dart
diff --git a/pkg/analyzer/test/src/context/mock_sdk.dart b/pkg/analyzer/test/src/context/mock_sdk.dart
index 96c66b26e9c1d7c3770289673cdfc938cfa47709..14861d8f4512d6cf90041d41aaaf4986ccd94408 100644
--- a/pkg/analyzer/test/src/context/mock_sdk.dart
+++ b/pkg/analyzer/test/src/context/mock_sdk.dart
@@ -276,6 +276,9 @@ abstract class Iterable<E> {
   void forEach(void f(E element));
 
   List<E> toList();
+
+  /*=R*/ fold/*<R>*/(/*=R*/ initialValue,
+      /*=R*/ combine(/*=R*/ previousValue, E element)) => null;
 }
 
 class List<E> implements Iterable<E> {
@@ -291,9 +294,6 @@ class List<E> implements Iterable<E> {
   E get first => null;
   E get last => null;
 
-  /*=R*/ fold/*<R>*/(/*=R*/ initialValue,
-      /*=R*/ combine(/*=R*/ previousValue, E element)) => null;
-
 }
 
 class Map<K, V> extends Object {