Add support for metadata annotation syntax (issue 4056)

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

 // Copyright (c) 2011, 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 com.google.dart.compiler.resolver.NodeElement;
 
 /**
  * Base class for directives.
  */
-public abstract class DartDirective extends DartNode {
+public abstract class DartDirective extends DartNodeWithMetadata {
   @Override
   public NodeElement getElement() {
     throw new UnsupportedOperationException(getClass().getSimpleName());
   }
 }