Integration tests running on the flutter tool may not get killed properly on Windows

[DaveShuckerow]

Previously, integration tests would run the devtools app via dart pub.snapshot run instead of starting pub directly. This prevented Windows-based dart test runs getting killed, leaving the pub process behind.

Something similar might be needed when using flutter to run these tests.

Flutter's documentation talks about using snapshots to run the tools here: https://github.com/flutter/flutter/wiki/The-flutter-tool#debugging-the-flutter-command-line-tool