Update mock sdk to fix test

lib/src/checker/dart_sdk.dart

 // Copyright (c) 2015, 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.
 
 /// Common logic needed to provide a Dart SDK to the analyzer's resolver. This
 /// includes logic to determine where the sdk is located in the filesystem, and
 /// definitions to provide mock sdks.
 library dev_compiler.src.checker.dart_sdk;
 
 import 'package:analyzer/src/generated/engine.dart';
@@ skipping 126 matching lines @@
         class _Override { const _Override(); }
         const Object override = const _Override();
         class _Proxy { const _Proxy(); }
         const Object proxy = const _Proxy();
 
         class Iterable<E> {}
         class List<E> implements Iterable<E> {
           List([int length]);
           List.filled(int length, E fill);
         }
-        class Map<K, V> {}
+        class Map<K, V> {
+          Iterable<K> get keys {}
+        }
         ''',
   'dart:async': '''
         class Future<T> {
           Future then(callback) {}
         }
         class Stream<T> {}
   ''',
   'dart:html': '''
         library dart.html;
         class HtmlElement {}
         ''',
 };