dart2js: support tear-offs in the startup emitter.

tests/compiler/dart2js/mock_libraries.dart

Index: tests/compiler/dart2js/mock_libraries.dart
diff --git a/tests/compiler/dart2js/mock_libraries.dart b/tests/compiler/dart2js/mock_libraries.dart
index a9f96efcbf7c5edca4c33135e8f3b57b756235a1..c823f6bbfba7e2ba2678b77230cc781710a2d1e4 100644
--- a/tests/compiler/dart2js/mock_libraries.dart
+++ b/tests/compiler/dart2js/mock_libraries.dart
@@ -196,6 +196,7 @@ const Map<String, String> DEFAULT_JS_HELPER_LIBRARY = const <String, String>{
   'patch': 'const patch = const _Patch(null);',
   'patch_full': 'const patch_full = const _Patch("full");',
   'patch_lazy': 'const patch_lazy = const _Patch("lazy");',
+  'patch_startup': 'const patch_startup = const _Patch("startup");',
   'propertyTypeCast': 'propertyTypeCast(x) {}',
   'propertyTypeCheck': 'propertyTypeCheck(value, property) {}',
   'requiresPreamble': 'requiresPreamble() {}',