Add optional message to assert in Dart2js.

pkg/compiler/lib/src/tree/prettyprint.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;
 
 /**
  * Pretty-prints Node tree in XML-like format.
  *
  * TODO(smok): Add main() to run from command-line to print out tree for given
@@ skipping 21 matching lines @@
     node.accept(p);
     return p.sb.toString();
   }
 
   visitNodeWithChildren(Node node, String type) {
     openNode(node, type);
     node.visitChildren(this);
     closeNode();
   }
 
+  visitAssert(Assert node) {
+    visitNodeWithChildren(node, "Assert");
+  }
+
   visitAsyncModifier(AsyncModifier node) {
     openAndCloseNode(node, "AsyncModifier",
         {'asyncToken': node.asyncToken,
          'starToken': node.starToken});
   }
 
   visitBlock(Block node) {
     visitNodeWithChildren(node, "Block");
   }
 
@@ skipping 398 matching lines @@
   }
 
   visitGotoStatement(GotoStatement node) {
     unimplemented('visitNode', node: node);
   }
 
   unimplemented(String message, {Node node}) {
     throw message;
   }
 }