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

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 fcaf75532475452fac77f871995bc0edf233356b..7d798c9ed3d26ed8e525c968ec717dddb2c01bb6 100644
--- a/pkg/analysis_server/test/domain_diagnostic_test.dart
+++ b/pkg/analysis_server/test/domain_diagnostic_test.dart
@@ -47,8 +47,7 @@ class DiagnosticDomainTest extends AbstractAnalysisTest {
     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);