Add visitors that throw by default

pkg/analyzer/lib/dart/ast/ast.dart

 // Copyright (c) 2014, 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.
 
 /**
  * Defines the AST model. The AST (Abstract Syntax Tree) model describes the
  * syntactic (as opposed to semantic) structure of Dart code. The semantic
  * structure of the code is modeled by the
  * [element model](../element/element.dart).
  *
@@ skipping 538 matching lines @@
   /**
    * Use the given [visitor] to visit all of the children of this node. The
    * children will be visited in lexical order.
    */
   void visitChildren(AstVisitor visitor);
 }
 
 /**
  * An object that can be used to visit an AST structure.
  *
- * Clients may extend or implement this class.
+ * Clients may not extend, implement or mix-in this class.

[Paul Berry, 2017/01/09 17:36:51]

Consider adding a reference to ThrowingAstVisitor here, so that clients who
previously extended or implemented this class will have a hint as to what they
should do instead.

[Brian Wilkerson, 2017/01/09 18:08:07]

Done

  */
 abstract class AstVisitor<R> {
   R visitAdjacentStrings(AdjacentStrings node);
 
   R visitAnnotation(Annotation node);
 
   R visitArgumentList(ArgumentList node);
 
   R visitAsExpression(AsExpression node);
 
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   /**
    * Return the 'yield' keyword.
    */
   Token get yieldKeyword;
 
   /**
    * Return the 'yield' keyword to the given [token].
    */
   void set yieldKeyword(Token token);
 }