Don't report redundant type errors in strong mode.

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

 // Copyright (c) 2014, 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.
 
 library analyzer.test.src.context.mock_sdk;
 
 import 'package:analyzer/file_system/file_system.dart' as resource;
 import 'package:analyzer/file_system/memory_file_system.dart' as resource;
 import 'package:analyzer/src/context/cache.dart';
 import 'package:analyzer/src/context/context.dart';
@@ skipping 133 matching lines @@
   Iterator<E> get iterator;
   bool get isEmpty;
   E get first;
 
   Iterable/*<R>*/ map/*<R>*/(/*=R*/ f(E e));
 
   /*=R*/ fold/*<R>*/(/*=R*/ initialValue,
       /*=R*/ combine(/*=R*/ previousValue, E element));
 }
 
-abstract class List<E> implements Iterable<E> {
-  void add(E value);
-  E operator [](int index);
-  void operator []=(int index, E value);
+class List<E> implements Iterable<E> {
+  List();
+  void add(E value) {}
+  E operator [](int index) => null;
+  void operator []=(int index, E value) {}
   Iterator<E> get iterator => null;
-  void clear();
+  void clear() {}
 }
 
-abstract class Map<K, V> extends Object {
-  Iterable<K> get keys;
-  V operator [](K key);
-  void operator []=(K key, V value);
+class Map<K, V> extends Object {
+  Iterable<K> get keys => null;
+  V operator [](K key) => null;
+  void operator []=(K key, V value) {}
 }
 
 external bool identical(Object a, Object b);
 
 void print(Object object) {}
 
 class _Override {
   const _Override();
 }
 const Object override = const _Override();
@@ skipping 205 matching lines @@
 
   @override
   AnalysisCache createCacheFromSourceFactory(SourceFactory factory) {
     if (factory == null) {
       return super.createCacheFromSourceFactory(factory);
     }
     return new AnalysisCache(
         <CachePartition>[AnalysisEngine.instance.partitionManager.forSdk(sdk)]);
   }
 }