Replaced use of lastIndexOf by indexOf because IE8 doesn't support it.

The effect in this situation is the same.
The use of one or the other is mostly a matter of efficiency.
Yet, the expected size of the indicators array makes efficiency not be so visible here.
The case where a given indicatorId is added more than once also is not affect by this change.
I made this change to be able to (finally) resolve http://jira.pentaho.com/browse/BISERVER-9147.