Add support for metadata annotation syntax (issue 4056)

compiler/java/com/google/dart/compiler/parser/CompletionHooksParserBase.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.parser;
 
 import com.google.dart.compiler.common.HasSourceInfo;
 
 /**
  * This class exists to enforce constraints on begin calls so code
@@ skipping 239 matching lines @@
   }
 
   protected void beginMapLiteral() {
     begin();
   }
 
   protected void beginMapLiteralEntry() {
     begin();
   }
 
+  protected void beginMetadata() {
+    begin();
+  }
+
   protected void beginMethodName() {
     begin();
   }
 
   protected void beginNativeBody() {
     begin();
   }
 
   protected void beginNativeDirective() {
     begin();
@@ skipping 179 matching lines @@
    * Begin a grammatical structure, saving the current location to later set in
    * an AST node. This may be followed by zero or more
    * {@link #doneWithoutConsuming(Object)} calls, and is terminated by exactly
    * one {@link #done(Object)} or {@link #rollback()} call.
    */
   private void begin() {
     assert guard.assertProgress();
     ctx.begin();
   }
 }