AnalysisOptionsProvider.getOptionsFile() should return only existing files.

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 c93e825ac96816ba4fa386a42c0ca3ed024807f9..913294f89463e929b550a4ad52b9b90357d01b4b 100644
--- a/pkg/analysis_server/test/domain_diagnostic_test.dart
+++ b/pkg/analysis_server/test/domain_diagnostic_test.dart
@@ -44,7 +44,7 @@ class DiagnosticDomainTest extends AbstractAnalysisTest {
     expect(context.name, '/project');
     expect(context.explicitFileCount, 1); /* test.dart */
 
-    expect(context.implicitFileCount, 5);
+    expect(context.implicitFileCount, 4);
 
     expect(context.workItemQueueLength, isNotNull);
   }