Add optional message to assert in Dart2js.

pkg/compiler/lib/src/tree/unparser.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.
 
 part of tree;
 
 String unparse(Node node, {minify: true}) {
   Unparser unparser = new Unparser(minify: minify);
   unparser.unparse(node);
   return unparser.result;
@@ skipping 40 matching lines @@
     sb.write(s);
     onEmptyLine = false;
   }
 
   unparse(Node node) { visit(node); }
 
   visit(Node node) {
     if (node != null) node.accept(this);
   }
 
+  visitAssert(Assert node) {
+    write(node.assertToken.value);
+    write('(');
+    visit(node.condition);
+    if (node.hasMessage) {
+      write(',');
+      space();
+      visit(node.message);
+    }
+    write(');');
+  }
+
   visitBlock(Block node) => unparseBlockStatements(node.statements);
 
   unparseBlockStatements(NodeList statements) {
     addToken(statements.beginToken);
 
     Link<Node> nodes = statements.nodes;
     if (nodes != null && !nodes.isEmpty) {
       indentMore();
       newline();
       visit(nodes.head);
@@ skipping 779 matching lines @@
   }
 
   visitStatement(Statement node) {
     throw 'internal error'; // Should not be called.
   }
 
   visitStringNode(StringNode node) {
     throw 'internal error'; // Should not be called.
   }
 }