Create a custom VM service extension and use it to implement the screenshot taking feature

[bartekpacia]

Idea was sparked by @jakubfijalkowski in this comment.

[bartekpacia]

Currently, I'm registering the service extension in the test and calling from the driver, but I want it the reverse.

Will poke around with this tomorrow.

[bartekpacia]

Works. Wow.

Caveats:

• conflicts with Use flutter test instead of flutter drive #421, because flutter test doesn't call the test_driver/integration_test.dart file.

  Possible (yet hacky workaround): we could manually rewrite the autogenerated listener.dart file to include functionality we need. See [integration_test] allow custom binding initialization when running integration_test tests flutter/flutter#113097 for more details.

• gotta find a way to write tests for this feature. Should take a look at how Flutter does it

• HostAutomator.takeScreenshot() will crash when more than single device is attached fixed

[shilangyu]

Usually assert messages say what was expected, explain the needed invariants (for instance 'host getter can be used only if hostAutomator was initialized before' or something). Because currently the error message is not helpful.

[bartekpacia]

Thanks, I agree.

[bartekpacia]

In the end, I improved the doc comment above native and host fields instead of improving the assertion message.

Albert told me some time ago that assertions should be in the form of:

X is null, Y is not in the inclusive 1..10 range

instead of:

X must not be null, Y must be in the inclusive 1..10 range

[shilangyu]

Albert told me some time ago that assertions should be in the form of:

X is null, Y is not in the inclusive 1..10 range

instead of:

X must not be null, Y must be in the inclusive 1..10 range

I cannot say I agree... But even then, I don't think such error messages are helpful at all. They don't give any meaningful information. I know what the value should or should not be, I can see what you are asserting (dart's assert is not a function, it is a language feature, they capture the expression and print the exact expression that was used to perform the assert, see attached screenshot). Asserts are for enforcing invariants so the error message should explain what kind of invariant was not fulfilled. Moving this to docs is great, though.

Coming back to this specific case, as an outside person who read the docs above this getter I still have no idea what should I do if I encounter a null here. Why would it be not initialized? Is it allowed to be not initialized? It is a final value, so I don't think I can initialize it myself.

[bartekpacia]

I really want do write tests for this, but it will happen in another PR.