Add optional message to assert in Dart2js.

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 137245849102c86a0b01edc0a3d5f536e75bcab5..25ca4645eacb0f60cae2a5fd23be842710b7097a 100644
--- a/tests/compiler/dart2js/mock_libraries.dart
+++ b/tests/compiler/dart2js/mock_libraries.dart
@@ -100,6 +100,8 @@ import 'dart:async';
 ''';
 
 const Map<String, String> DEFAULT_JS_HELPER_LIBRARY = const <String, String>{
+  'assertTest': 'assertTest(a) {}',
+  'assertThrow': 'assertThrow(a) {}',
   'assertHelper': 'assertHelper(a) {}',
   'assertIsSubtype': 'assertIsSubtype(subtype, supertype, message) {}',
   'assertSubtype': 'assertSubtype(object, isField, checks, asField) {}',
@@ -384,7 +386,7 @@ const Map<String, String> DEFAULT_ISOLATE_HELPER_LIBRARY =
 
 const Map<String, String> DEFAULT_ASYNC_LIBRARY = const <String, String>{
   'DeferredLibrary': 'class DeferredLibrary {}',
-  'Future': 
+  'Future':
       '''
       class Future<T> {
         Future.value([value]);