Add inspector/diagnostics.dart and split.dart exports to devtools_app.dart (flutter#3469)

Author: matt-ragonese

packages/devtools_app/lib/devtools_app.dart

@@ -18,6 +18,7 @@ export 'src/debugger/syntax_highlighter.dart';
 export 'src/error_badge_manager.dart';
 export 'src/globals.dart';
 export 'src/http/http_request_data.dart';
+export 'src/inspector/diagnostics.dart';
 export 'src/inspector/diagnostics_node.dart';
 export 'src/inspector/inspector_controller.dart';
 export 'src/inspector/inspector_service.dart';
@@ -40,6 +41,7 @@ export 'src/routing.dart';
 export 'src/screen.dart';
 export 'src/service_extensions.dart';
 export 'src/service_manager.dart';
+export 'src/split.dart';
 export 'src/theme.dart';
 export 'src/trace_event.dart';
 export 'src/ui/icons.dart';