fixes #25793, make strong checker tests sane

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 18 matching lines @@
 
 class Function {}
 class StackTrace {}
 class Symbol {}
 class Type {}
 
 abstract class Comparable<T> {
   int compareTo(T other);
 }
 
-abstract class String implements Comparable<String> {
+abstract class Pattern {}
+abstract class String implements Comparable<String>, Pattern {
   external factory String.fromCharCodes(Iterable<int> charCodes,
                                         [int start = 0, int end]);
   String operator +(String other) => null;
   bool get isEmpty => false;
   bool get isNotEmpty => false;
   int get length => 0;
   String substring(int len) => null;
   String toUpperCase();
   List<int> get codeUnits;
 }
@@ skipping 314 matching lines @@
 
   @override
   AnalysisCache createCacheFromSourceFactory(SourceFactory factory) {
     if (factory == null) {
       return super.createCacheFromSourceFactory(factory);
     }
     return new AnalysisCache(
         <CachePartition>[AnalysisEngine.instance.partitionManager.forSdk(sdk)]);
   }
 }