title: firebase Certificate object must contain a string "private_key" property
body: <p>I am using firebase cloud messaging with node.js. I just want to connect to  firebase cloud messaging 
  I get this error message:</p>
  
  <pre><code>/home/t/ws/js/webserver/blitzer/node_modules/firebase-admin/lib/auth/credential.js:119
              throw new error_1.FirebaseAppError(error_1.AppErrorCodes.INVALID_CREDENTIAL, errorMessage);
              ^
  
  Error: Certificate object must contain a string "private_key" property.
  </code></pre>
  
  <p>this is my code: </p>
  
  <pre><code>import * as admin from 'firebase-admin'
  import serviceAccount from  './google-services.json'
  const registrationToken = 'fMaw-YgWgwY:APA91bEwv7Z_FHPx0kCuiM6_Ji004_d_K5WQh9PMH1IgshLb_Lqq7zhicKdg6lFO5dl0FY3im5r0jgMZrWNbZIXpPL4k1JC22hT5fUY5h5B3RVXjgRCBQI-l9cNxBRVZP-cnJtyYbqMKqmwvGD8fg3ae8QQjAkRe0w'
  
  var message = {
      data: {
        score: '850',
        time: '2:45'
      },
      token: registrationToken
    };
  admin.initializeApp({
      credential: admin.credential.cert(serviceAccount)
  });
  
  
  admin.messaging().send(message)
    .then((response) =&gt; {
      // Response is a message ID string.
      console.log('Successfully sent message:', response);
    })
    .catch((error) =&gt; {
      console.log('Error sending message:', error);
    });
  </code></pre>
  
  <p>and here is my google-services.json file. I downloaded it from firebase console. There is no key private_key property that is right, but I have downloaded the correct google-services.json. My Firebase-admin version is 5.13.0</p>
  
  <pre><code>{
    "project_info": {
      "project_number": "x",
      "firebase_url": "x",
      "project_id": "x",
      "storage_bucket": "x"
    },
    "client": [
      {
        "client_info": {
          "mobilesdk_app_id": "x",
          "android_client_info": {
            "package_name": "x"
          }
        },
        "oauth_client": [
          {
            "client_id": "x",
            "client_type": x
          }
        ],
        "api_key": [
          {
            "current_key": "x"
          }
        ],
        "services": {
          "analytics_service": {
            "status": x
          },
          "appinvite_service": {
            "status": x,
            "other_platform_oauth_client": []
          },
          "ads_service": {
            "status": x
          }
        }
      }
    ],
    "configuration_version": "x"
  }
  </code></pre>