The dropIndexes command must take an array of indexes to drop for a single collection.
This is related to
SERVER-37726, which will add the ability to drop in-progress index builds. A createIndexes command can build several indexes under one builder, and right now we can only abort at the builder granularity. So dropIndexes will only be able to drop all or none of the indexes associated with a builder. Thus dropIndexes must take multiple indexes as an argument.