Add optional message to assert in Dart2js.

tests/compiler/dart2js/backend_dart/dart_printer_test.dart

Index: tests/compiler/dart2js/backend_dart/dart_printer_test.dart
diff --git a/tests/compiler/dart2js/backend_dart/dart_printer_test.dart b/tests/compiler/dart2js/backend_dart/dart_printer_test.dart
index 3ab7e86e720c3b051ce1a4a6c053d0413c1b0b08..1742c23a7adae9fea9bfea99383fdffa95eecc53 100644
--- a/tests/compiler/dart2js/backend_dart/dart_printer_test.dart
+++ b/tests/compiler/dart2js/backend_dart/dart_printer_test.dart
@@ -583,9 +583,14 @@ class AstBuilder extends Listener {
     push(new ForIn(declaredIdentifier, exp, body));
   }
 
-  handleAssertStatement(Token assertKeyword, Token semicolonToken) {
+  handleAssertStatement(Token assertKeyword,
+                        Token commaToken, Token semicolonToken) {
+    Expression message;
+    if (commaToken != null) message = pop();
     Expression exp = pop();
-    Expression call = new CallFunction(new Identifier("assert"), [exp]);
+    var arguments = [exp];
+    if (message != null) arguments.add(message);
+    Expression call = new CallFunction(new Identifier("assert"), arguments);
     push(new ExpressionStatement(call));
   }