Don't add sources to DartSdk contexts.

pkg/analysis_server/test/domain_diagnostic_test.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.
 
 library test.domain.diagnostic;
 
 import 'package:analysis_server/plugin/protocol/protocol.dart';
 import 'package:analysis_server/src/analysis_server.dart';
 import 'package:analysis_server/src/domain_diagnostic.dart';
 import 'package:analysis_server/src/plugin/server_plugin.dart';
@@ skipping 53 matching lines @@
       resourceProvider.newFile('/project/pubspec.yaml', 'name: project');
       resourceProvider.newFile(file, 'main() {}');
 
       server.setAnalysisRoots('0', ['/project/'], [], {});
 
       await server.onAnalysisComplete;
 
       var request = new DiagnosticGetDiagnosticsParams().toRequest('0');
       var response = handler.handleRequest(request);
 
-      int fileCount = MockSdk.LIBRARIES.length + 1 /* test.dart */;
+      int fileCount = 1 /* test.dart */;
 
       var json = response.toJson()[Response.RESULT];
       expect(json['contexts'], hasLength(1));
       var context = json['contexts'][0];
       expect(context['name'], '/project');
       expect(context['explicitFileCount'], fileCount);
-      expect(context['implicitFileCount'], 0);
+      // dart:core dart:async dart:math dart:_internal
+      expect(context['implicitFileCount'], 4);
       expect(context['workItemQueueLength'], isNotNull);
     });
 
     test('getDiagnostics - (no root)', () async {
       var request = new DiagnosticGetDiagnosticsParams().toRequest('0');
       var response = handler.handleRequest(request);
       var json = response.toJson()[Response.RESULT];
       expect(json['contexts'], hasLength(0));
     });
   });
 }