Extract 'DartUnitHighlightsComputer' tests into a separate file.

pkg/analysis_server/test/analysis_notification_outline_test.dart

Index: pkg/analysis_server/test/analysis_notification_outline_test.dart
diff --git a/pkg/analysis_server/test/analysis_notification_outline_test.dart b/pkg/analysis_server/test/analysis_notification_outline_test.dart
index c0a7024fb09fce613a5456f0af687f9a8e456bca..0becad14d18ea3c40f26b85e4d2476513d0c8a52 100644
--- a/pkg/analysis_server/test/analysis_notification_outline_test.dart
+++ b/pkg/analysis_server/test/analysis_notification_outline_test.dart
@@ -18,13 +18,13 @@ import 'reflective_tests.dart';
 
 main() {
   group('notification.outline', () {
-    runReflectiveTests(AnalysisNotificationOutlineTest);
+    runReflectiveTests(_AnalysisNotificationOutlineTest);
   });
 }
 
 
 @ReflectiveTestCase()
-class AnalysisNotificationOutlineTest extends AbstractAnalysisTest {
+class _AnalysisNotificationOutlineTest extends AbstractAnalysisTest {
   _Outline outline;
 
   @override