title: Swift compiler Error "failed with exit code 254"
body: <p>When I update Xcode 6 from Beta 4 to the Beta 6, I have this (new) compiler error and I don't know how to fix it.</p>
  
  <pre><code>&lt;unknown&gt;:0: error: unable to execute command: Segmentation fault: 11
  &lt;unknown&gt;:0: error: swift frontend command failed due to signal (use -v to see invocation)
  Command /Applications/Xcode6-Beta6.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/swiftc failed with exit code 254
  </code></pre>
  
  <p>The concerned lines are these</p>
  
  <pre><code>let reuseId = "test"
  
      var anView = mapView.dequeueReusableAnnotationViewWithIdentifier(reuseId)
      if anView == nil{...}
  </code></pre>
  
  <p>Thanks.</p>