This can't be made `protected` because it's defined in the `MapOutputTracker` object but used in `MapOutputTrackerMaster`, which isn't a subclass of the companion object:

```scala
[error] /Users/joshrosen/Documents/spark/spark/core/src/main/scala/org/apache/spark/MapOutputTracker.scala:254: method serializeMapStatuses in object MapOutputTracker cannot be accessed in object org.apache.spark.MapOutputTracker
[error]  Access to protected method serializeMapStatuses not permitted because
[error]  enclosing class class MapOutputTrackerMaster in package spark is not a subclass of
[error]  object MapOutputTracker in package spark where target is defined
[error]     val bytes = MapOutputTracker.serializeMapStatuses(statuses)
```