Add optional message to assert in Dart2js.

pkg/compiler/lib/src/inferrer/inferrer_visitor.dart

 // Copyright (c) 2013, 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.
 
 library inferrer_visitor;
 
 import 'dart:collection' show
     IterableMixin;
 
 import '../compiler.dart' show
@@ skipping 764 matching lines @@
   @override
   T bulkHandlePrefix(SendSet node, _) {
     return handleSendSet(node);
   }
 
   @override
   T bulkHandlePostfix(SendSet node, _) {
     return handleSendSet(node);
   }
 
-  @override
-  T visitAssert(Send node, Node expression, _) {
-    if (!compiler.enableUserAssertions) {
-      return types.nullType;
-    }
-    return handleAssert(node, expression);
-  }
-
-  /// Handle an enabled assertion of [expression].
-  T handleAssert(Send node, Node expression);
-
   T visitNode(Node node) {
     return node.visitChildren(this);
   }
 
   T visit(Node node) {
     return node == null ? null : node.accept(this);
   }
 
   T visitFunctionDeclaration(FunctionDeclaration node) {
     locals.update(elements[node], types.functionType, node);
@@ skipping 716 matching lines @@
     return type;
   }
 
   T visitCascade(Cascade node) {
     // Ignore the result of the cascade send and return the type of the cascade
     // receiver.
     visit(node.expression);
     return cascadeReceiverStack.removeLast();
   }
 }