Update context manager test after turning DOWN_CAST_COMPOSITE strong mode warning into a hint.

pkg/analysis_server/test/context_manager_test.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 test.context.directory.manager;
 
 import 'dart:collection';
 
 import 'package:analysis_server/src/context_manager.dart';
 import 'package:analysis_server/src/utilities/null_string_sink.dart';
@@ skipping 2313 matching lines @@
       expect(sdkContext.analysisOptions.strongMode, isTrue);
       // The code is strong-mode clean.
       // Verify that TypeSystem was reset.
       expect(result.errors, isEmpty);
     } else {
       AnalysisContext context = manager.getContextFor(projPath);
       Source testSource = context.getSourcesWithFullName(file.path).single;
       AnalysisContext sdkContext = sourceFactory.dartSdk.context;
       expect(analysisOptions.strongMode, isTrue);
       expect(sdkContext.analysisOptions.strongMode, isTrue);
-      // The code is strong-mode clean, but we're using a Mock SDK that isn't
-      // configured correctly for strong mode so we get an error.
+      // The code is strong-mode clean.
       // Verify that TypeSystem was reset.
-      expect(context.computeErrors(testSource), hasLength(1));
+      expect(context.computeErrors(testSource), isEmpty);
     }
   }
 
   test_path_filter_analysis_option() async {
     // Create files.
     String libPath = newFolder([projPath, ContextManagerTest.LIB_NAME]);
     newFile([libPath, 'main.dart']);
     newFile([libPath, 'nope.dart']);
     String sdkExtPath = newFolder([projPath, 'sdk_ext']);
     newFile([sdkExtPath, 'entry.dart']);
@@ skipping 549 matching lines @@
 class TestUriResolver extends UriResolver {
   Map<Uri, Source> uriMap;
 
   TestUriResolver(this.uriMap);
 
   @override
   Source resolveAbsolute(Uri uri, [Uri actualUri]) {
     return uriMap[uri];
   }
 }