Isolate-based access to the SDK index (but not used yet).

[isoos]

• SDK search index improvements (tracking issue) #8670
• refactored and reused the package index's code into a bit more generic runIsolateFunctions to handle the initialization and message passing on the service isolate side
• added a test that runs the SDK index in an isolate, will update the search service to use it in a subsequent PR

[sigurdm]

Code looks OK, but I'm not sure why we want this...
Is it for parallel processing?

[isoos]

Yes, the goal would be to run the SDK index search on a different isolate, in the hope that the overall latency could go down.

I think it is worth to merge this (for the _isolate.dart refactor is useful on its own), and as a next step, I will do a local benchmark with one vs two separate isolates. I think the overhead won't be that much compared to the latency gains, and on our search instances we have 2 CPUs, one of which is mostly idle (except for the time when the isolate refresh happens).