Add support for metadata annotation syntax (issue 4056)

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

Index: compiler/java/com/google/dart/compiler/ast/ASTVisitor.java
===================================================================
--- compiler/java/com/google/dart/compiler/ast/ASTVisitor.java	(revision 10903)
+++ compiler/java/com/google/dart/compiler/ast/ASTVisitor.java	(working copy)
@@ -237,6 +237,10 @@
     return visitNode(node);
   }
 
+  public R visitMetadata(DartMetadata node) {
+    return visitNode(node);
+  }
+
   public R visitMethodDefinition(DartMethodDefinition node) {
     return visitClassMember(node);
   }