While we are messing with dagScheduler, I'd prefer that it not be mutable everywhere within the spark namespace.  It looks like it is that way purely for testing purposes, and that it can be turned into a private val while only exposing a mutable Boolean flag to the rest of spark.

https://github.com/markhamstra/spark/compare/CodingCat:SPARK-1715...SPARK-1715?expand=1