Setting flag `pause_isolates_on_start` does not pause isolates on hot-restart

[elliette]

The VM Service allows clients to set pause_isolates_on_start at runtime:

• https://api.flutter.dev/flutter/vm_service/VmServiceInterface/setFlag.html

Here's an example of doing so in Dart DevTools:

• https://github.com/flutter/devtools/compare/master...elliette:pause-isolates-on-start

However, with that change, the isolates are not paused on hot-restart.

Steps to repro:

• patch the change into DevTools
• run DevTools and connect to an app
• trigger a hot-restart from DevTools
• isolates do not start paused

Expected result: isolates should start paused

FYI @bkonyi