dropIndexes should abort in-progress index builds. This should be done after
SERVER-37763 makes it possible to abort an index build. SERVER-37727 will enhance dropIndexes to receive an array of indexes as arguments. This task will only abort in-progress index builds if the user specifies all of the indexes that a single builder is building together. A createIndexes command can start multiple indexes building together on one builder and we currently only have the granularity to abort all or none of the indexes on a builder.
Lastly, dropIndexes will not write a dropIndexes oplog entry if aborting in-progress builds. Aborting the index will produce an abortIndexBuilds oplog entry, which suffices. This allows rollback via refetch to know that an index was fully built prior to a dropIndexes oplog entry.