Add Iterable type to mock SDK for analyzer2dart.

pkg/analyzer2dart/test/mock_sdk.dart

Index: pkg/analyzer2dart/test/mock_sdk.dart
diff --git a/pkg/analyzer2dart/test/mock_sdk.dart b/pkg/analyzer2dart/test/mock_sdk.dart
index 221d02e76d6ad9ea4547860b1606b9b8ed793357..8cebd44181f4ec79dc144f3fb7724cbe96e52199 100644
--- a/pkg/analyzer2dart/test/mock_sdk.dart
+++ b/pkg/analyzer2dart/test/mock_sdk.dart
@@ -58,7 +58,9 @@ class Deprecated extends Object {
 }
 const Object deprecated = const Deprecated("next release");
 
-abstract class List<E> extends Object {
+abstract class Iterable<E> {}
+
+abstract class List<E> extends Object implements Iterable {
   void add(E value);
   E operator [](int index);
   void operator []=(int index, E value);