Don't create FileState(s) for SDK source in _ContentCacheWrapper.

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/domain_diagnostic.dart';
 import 'package:test/test.dart';
 import 'package:test_reflective_loader/test_reflective_loader.dart';
@@ skipping 29 matching lines @@
     var response = handler.handleRequest(request);
     var result = new DiagnosticGetDiagnosticsResult.fromResponse(response);
 
     expect(result.contexts, hasLength(1));
 
     ContextData context = result.contexts[0];
     expect(context.name, '/project');
     expect(context.explicitFileCount, 1); /* test.dart */
 
     if (enableNewAnalysisDriver) {
-      // dart:core (although it should not be here)
-      expect(context.implicitFileCount, 1);
+      expect(context.implicitFileCount, 0);
     } else {
       // dart:core dart:async dart:math dart:_internal
       expect(context.implicitFileCount, 4);
     }
 
     expect(context.workItemQueueLength, isNotNull);
   }
 
   test_getDiagnostics_noRoot() async {
     var request = new DiagnosticGetDiagnosticsParams().toRequest('0');
     var response = handler.handleRequest(request);
     var result = new DiagnosticGetDiagnosticsResult.fromResponse(response);
     expect(result.contexts, isEmpty);
   }
 }
 
 @reflectiveTest
 class DiagnosticDomainTest_Driver extends DiagnosticDomainTest {
   @override
   void setUp() {
     enableNewAnalysisDriver = true;
     generateSummaryFiles = true;
     super.setUp();
   }
 }