Add optional message to assert in Dart2js.

pkg/compiler/lib/src/scanner/listener.dart

Index: pkg/compiler/lib/src/scanner/listener.dart
diff --git a/pkg/compiler/lib/src/scanner/listener.dart b/pkg/compiler/lib/src/scanner/listener.dart
index c3299a3c5eed23f18200e199e261c94cf84b49ea..6cc299822c1ab100c3339b572af0b940e31d37a7 100644
--- a/pkg/compiler/lib/src/scanner/listener.dart
+++ b/pkg/compiler/lib/src/scanner/listener.dart
@@ -472,7 +472,8 @@ class Listener {
   void handleEmptyStatement(Token token) {
   }
 
-  void handleAssertStatement(Token assertKeyword, Token semicolonToken) {
+  void handleAssertStatement(Token assertKeyword,
+                             Token commaToken, Token semicolonToken) {
   }
 
   /** Called with either the token containing a double literal, or
@@ -2210,11 +2211,16 @@ class NodeListener extends ElementListener {
     }
   }
 
-  void handleAssertStatement(Token assertKeyword, Token semicolonToken) {
-    NodeList arguments = popNode();
-    Node selector = new Identifier(assertKeyword);
-    Node send = new Send(null, selector, arguments);
-    pushNode(new ExpressionStatement(send, semicolonToken));
+  void handleAssertStatement(Token assertKeyword,
+                             Token commaToken, Token semicolonToken) {
+    Node message;
+    Node condition;
+    if (commaToken != null) {
+      message = popNode();
+    }
+    condition = popNode();
+    pushNode(new Assert(assertKeyword, condition,
+                        message, semicolonToken));
   }
 
   void endUnnamedFunction(Token token) {