title: Can't import AppCompatActivity
body: <p>I was transitioning my app from GCM to Firebase for push notifications which involved changing some dependencies. I changed the gcm dependency to firebase and tried to sync the projec, but after syncing I found that all my classes which had implemented AppCompatActivity were unable to resolve AppCompatActivity, and the import statement was greyed out.</p>
  
  <p><a href="https://i.stack.imgur.com/fj61q.png" rel="nofollow"><img src="https://i.stack.imgur.com/fj61q.png" alt="enter image description here"></a></p>
  
  <p>I've never had problems with AppCompatActivity before, and I know AndroidStudio can be buggy sometimes so I'm assuming this is an AS issue rather than something I'm doing wrong. Here are my dependencies:</p>
  
  <pre><code>dependencies {
      compile fileTree(include: ['*.jar'], dir: 'libs')
      compile 'com.android.support:appcompat-v7:21.0.3'
      compile 'com.android.support:recyclerview-v7:21.0.3'
      compile 'com.android.support:cardview-v7:21.0.+'
      compile 'com.mcxiaoke.volley:library:1.0.+'
      compile 'com.facebook.android:facebook-android-sdk:4.1.0'
      compile 'com.google.firebase:firebase-messaging:9.0.0'
  }
  </code></pre>
  
  <p>I've tried cleaning and rebuilding the project multiple times, restarting AS... no luck.</p>