Add optional message to assert in Dart2js.

pkg/compiler/lib/src/parser/parser.dart

 // Copyright (c) 2012, 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 dart2js.parser;
 
 import '../diagnostics/messages.dart' show
     MessageKind;
 import '../tokens/keyword.dart' show
     Keyword;
@@ skipping 2679 matching lines @@
     if (token.isIdentifier()) {
       token = parseIdentifier(token);
       hasTarget = true;
     }
     listener.handleBreakStatement(hasTarget, breakKeyword, token);
     return expectSemicolon(token);
   }
 
   Token parseAssertStatement(Token token) {
     Token assertKeyword = token;
+    Token commaToken = null;
     token = expect('assert', token);
-    expect('(', token);
-    token = parseArguments(token);
-    listener.handleAssertStatement(assertKeyword, token);
+    token = expect('(', token);
+    bool old = mayParseFunctionExpressions;
+    mayParseFunctionExpressions = true;
+    token = parseExpression(token);
+    if (optional(',', token)) {
+      commaToken = token;
+      token = token.next;
+      token = parseExpression(token);
+    }
+    token = expect(')', token);
+    mayParseFunctionExpressions = old;
+    listener.handleAssertStatement(assertKeyword, commaToken, token);
     return expectSemicolon(token);
   }
 
   Token parseContinueStatement(Token token) {
     assert(optional('continue', token));
     Token continueKeyword = token;
     token = token.next;
     bool hasTarget = false;
     if (token.isIdentifier()) {
       token = parseIdentifier(token);
       hasTarget = true;
     }
     listener.handleContinueStatement(hasTarget, continueKeyword, token);
     return expectSemicolon(token);
   }
 
   Token parseEmptyStatement(Token token) {
     listener.handleEmptyStatement(token);
     return expectSemicolon(token);
   }
 }