Browser/VM debug modes should be enableable outside of --pause-after-load

[willdrach-wk]

As of right now, debug modes are controlled primarily by the --pause-after-load flag, and this will make it difficult in the future to control debug-related functionality, such as gathering code coverage. For this reason, I believe it would be best to decouple these modes from --pause-after-load in favor of a more adaptable API.

This could be done either just through the API, or could even be added to its own flag, such as --debug. I don't see much reason to not add it to its own flag, so that might be the better solution.

Here's where those debug modes are set:

• In vm/platform.dart
• In browser/platform.dart
• Again in browser/platform.dart
• In reporter/json.dart
• And in an important alert in runner.dart