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 801b75d1063d1f11d77c7ec2a556ec01cb067548..f6fd38bea1bf4c881d072e54887f4b89b43e688b 100644
--- a/tests/compiler/dart2js/backend_dart/dart_printer_test.dart
+++ b/tests/compiler/dart2js/backend_dart/dart_printer_test.dart
@@ -587,9 +587,14 @@ 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));
   }