Add optional message to assert in Dart2js.

pkg/compiler/lib/src/tree/unparser.dart

Index: pkg/compiler/lib/src/tree/unparser.dart
diff --git a/pkg/compiler/lib/src/tree/unparser.dart b/pkg/compiler/lib/src/tree/unparser.dart
index b7d31098a3a8f2845148da5adb0e795974602fb3..388546ac975f612c86108127a65595d6ab1848b8 100644
--- a/pkg/compiler/lib/src/tree/unparser.dart
+++ b/pkg/compiler/lib/src/tree/unparser.dart
@@ -58,6 +58,18 @@ class Unparser extends Indentation implements Visitor {
     if (node != null) node.accept(this);
   }
 
+  visitAssert(Assert node) {
+    write(node.assertToken.value);
+    write('(');
+    visit(node.condition);
+    if (node.message != null) {
+      write(',');
+      space();
+      visit(node.message);
+    }
+    write(');');
+  }
+
   visitBlock(Block node) => unparseBlockStatements(node.statements);
 
   unparseBlockStatements(NodeList statements) {