Add support for metadata annotation syntax (issue 4056)

compiler/java/com/google/dart/compiler/ast/ASTVisitor.java

 // 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.
 
 package com.google.dart.compiler.ast;
 
 import java.util.List;
 
 /**
  * A visitor for abstract syntax tree.
@@ skipping 59 matching lines @@
   }
 
   public R visitClassMember(DartClassMember<?> node) {
     return visitDeclaration(node);
   }
 
   public R visitComment(DartComment node) {
     return visitNode(node);
   }
 
+  public R visitAnnotation(DartAnnotation node) {
+    return visitNode(node);
+  }
+
   public R visitArrayAccess(DartArrayAccess node) {
     return visitExpression(node);
   }
 
   public R visitArrayLiteral(DartArrayLiteral node) {
     return visitTypedLiteral(node);
   }
 
   public R visitBinaryExpression(DartBinaryExpression node) {
     return visitExpression(node);
@@ skipping 280 matching lines @@
   }
 
   public R visitNativeBlock(DartNativeBlock node) {
     return visitBlock(node);
   }
 
   public R visitRedirectConstructorInvocation(DartRedirectConstructorInvocation node) {
     return visitInvocation(node);
   }
 }