Don't add sources to DartSdk contexts.

pkg/analysis_server/test/domain_diagnostic_test.dart

Index: pkg/analysis_server/test/domain_diagnostic_test.dart
diff --git a/pkg/analysis_server/test/domain_diagnostic_test.dart b/pkg/analysis_server/test/domain_diagnostic_test.dart
index f234463336dbab1ae88a68887ac7de9d32c3129f..e0a6867b8ce54b2a600aed2d6fb4d695f4e33fd5 100644
--- a/pkg/analysis_server/test/domain_diagnostic_test.dart
+++ b/pkg/analysis_server/test/domain_diagnostic_test.dart
@@ -71,14 +71,15 @@ main() {
       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);
     });