Add optional message to assert in Dart2js.

tests/compiler/dart2js/backend_dart/dart_printer_test.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library dart_printer_test;
 
 import 'dart:mirrors';
 import 'package:expect/expect.dart';
 import 'package:compiler/src/constants/values.dart';
 import 'package:compiler/src/dart_backend/backend_ast_nodes.dart';
@@ skipping 565 matching lines @@
     push(null);
   }
 
   endForIn(Token await, Token begin, Token inKeyword, Token end) {
     Statement body = pop();
     Expression exp = pop();
     Node declaredIdentifier = pop();
     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));
   }
 
   endLabeledStatement(int labelCount) {
     Statement statement = pop();
     for (int i=0; i<labelCount; i++) {
       String label = pop(asName);
       statement = new LabeledStatement(label, statement);
     }
     push(statement);
@@ skipping 371 matching lines @@
   checkStatement("do while(x); while (y);");
   checkStatement("{do; while(x); while (y);}");
 
   checkStatement('switch(x) { case 1: case 2: return y; }');
   checkStatement('switch(x) { default: return y; }');
   checkStatement('switch(x) { case 1: x=y; default: return y; }');
   checkStatement('switch(x) { case 1: x=y; y=z; break; default: return y; }');
 
 }