Add --debug-builders flag

[simolus3]

With recent changes to the bootstrapping logic, builders have become almost impossible to debug in real projects:

• Running build_runner itself with --observe doesn't do much because most of the build happens in a sub-process.
• The sub-process can't be started manually because it needs state sent to it from stdin, which is tricky to do when launching it from editors.

I've also started a discussion on this: #4242. Originally I wanted to wait for feedback but this turned out to be fairly simple in the end, so here's a PR :)

This adds the --debug-builders flag. When set, the subprocess running the build will launch with --observe and --pause-isolates-on-start. This makes Dart print out the VM service and devtools URLs, so one can attach a debugger, set breakpoints and then resume the build.

I've tested these changes manually, I'm not sure how a good automated test for this could look like. Start a build, assert that a "VM service URL" line is printed on stdout and then kill it?

[davidmorgan]

Thanks!

I already felt the need for this myself :)

What do you think about instead of --debug-builders, having a multi-option --dart-arg that just passes through anything to dart run before the script? Then you would do --dart-arg=--observe --dart-arg=--pause-isolates-on-start or anything else you want. It can be wired up pretty much exactly as you already have it.

When AOT compile is added I guess the same flags won't apply to both, so maybe the name should be more explicit ... maybe --dart-jit-vm-arg?

For testing, it's easy to do a test that runs, checks output and kills using a recently-added test type, it would be like the "watch" test because that deals with a running process then killing it, too

build/build_runner/test/integration_tests/watch_command_test.dart

Line 34 in 9f07c7e

var watch = await tester.start('root_pkg', 'dart run build_runner watch');

I suggest adding a new test named something like build_command_vm_arg_test.dart.

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[davidmorgan]

Thanks :) I guess I'll get to releasing a new version some time next week, feel free to nudge me if it starts to be too long :)