Add optional message to assert in Dart2js.

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

Index: pkg/compiler/lib/src/scanner/parser.dart
diff --git a/pkg/compiler/lib/src/scanner/parser.dart b/pkg/compiler/lib/src/scanner/parser.dart
index 66d26bdf5cb0ae003c8eaaeeecbd59e0fb01fd59..69d4e62feb4277ad9d4395c98865e910e1361327 100644
--- a/pkg/compiler/lib/src/scanner/parser.dart
+++ b/pkg/compiler/lib/src/scanner/parser.dart
@@ -2639,10 +2639,20 @@ class Parser {
 
   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);
   }