Add support for metadata annotation syntax (issue 4056)

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

Index: compiler/java/com/google/dart/compiler/ast/DartNodeWithMetadata.java
===================================================================
--- compiler/java/com/google/dart/compiler/ast/DartNodeWithMetadata.java	(revision 0)
+++ compiler/java/com/google/dart/compiler/ast/DartNodeWithMetadata.java	(revision 0)
@@ -0,0 +1,32 @@
+// 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;
+
+/**
+ * The abstract class {@code DartNodeWithMetadata} defines the behavior of nodes that can have
+ * metadata associated with them.
+ */
+public abstract class DartNodeWithMetadata extends DartNode {
+  private NodeList<DartAnnotation> metadata = NodeList.create(this);
+
+  protected DartNodeWithMetadata() {
+    super();
+  }
+
+  public NodeList<DartAnnotation> getMetadata() {
+    return metadata;
+  }
+
+  public void setMetadata(List<DartAnnotation> metadata) {
+    this.metadata.addAll(metadata);
+  }
+
+  @Override
+  public void visitChildren(ASTVisitor<?> visitor) {
+    metadata.accept(visitor);
+  }
+}