This is just to try and reduce the stack tract text, which can be several thousand chars long, to something that can meaningfully fit into the analytics 100 char length limit. I want those 100 chars to be something that we could use to try and diagnose exceptions that come in. I'll add some dartdoc to that effect.

I don't think this will have PII concerns; the stack trace will always be for dart: libraries, or Spark code that we wrote.