code: function extractPlaceholders(messageBundle) {
  	      var messageMap = messageBundle.getMessageMap();
  	      var placeholders = {};
  	      Object.keys(messageMap).forEach(function (msgId) {
  	          placeholders[msgId] = messageMap[msgId].placeholders;
  	      });
  	      return placeholders;
  	  }
docs: @license
  Copyright Google Inc. All Rights Reserved.
  
  Use of this source code is governed by an MIT-style license that can be
  found in the LICENSE file at https://angular.io/license
   Generate a map of placeholder to content indexed by message ids