Add Dart code to diff_view

pkg/js_ast/lib/src/nodes.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 js_ast;
 
 abstract class NodeVisitor<T> {
   T visitProgram(Program node);
 
   T visitBlock(Block node);
@@ skipping 63 matching lines @@
 
   T visitInterpolatedExpression(InterpolatedExpression node);
   T visitInterpolatedLiteral(InterpolatedLiteral node);
   T visitInterpolatedParameter(InterpolatedParameter node);
   T visitInterpolatedSelector(InterpolatedSelector node);
   T visitInterpolatedStatement(InterpolatedStatement node);
   T visitInterpolatedDeclaration(InterpolatedDeclaration node);
 }
 
 class BaseVisitor<T> implements NodeVisitor<T> {
+  const BaseVisitor();
+
   T visitNode(Node node) {
     node.visitChildren(this);
     return null;
   }
 
   T visitProgram(Program node) => visitNode(node);
 
   T visitStatement(Statement node) => visitNode(node);
   T visitLoop(Loop node) => visitStatement(node);
   T visitJump(Statement node) => visitStatement(node);
@@ skipping 1193 matching lines @@
 class Comment extends Statement {
   final String comment;
 
   Comment(this.comment);
 
   accept(NodeVisitor visitor) => visitor.visitComment(this);
   Comment _clone() => new Comment(comment);
 
   void visitChildren(NodeVisitor visitor) {}
 }