[breaking change] Make `dart:io` `Platform` `abstract`

[brianquinlan]

Change Intent

Add the abstract class modifier to the dart:io Platform class.

Justification

Platform only has static properties so there is no reason to make instances of it.

Impact

All code that constructs Platform (i.e. Platform()) will break. There is one known pattern where this is done: package:dcli implements a non-static extension on Platform to allow users to get the line ending for the platform i.e.

Platform().eol;

Mitigation

In Dart 3.1:

• The constructor for Platform is deprecated.
• Platform contains a lineTerminator property that obviates the need for the extension in package:dcli.

Before we mark Platform as abstract, I will remove the extension from package:dcli and give users some time to update their dependencies. Making the change in Dart 3.2 would give users an approximately 6 week period to update their dependencies and code.

[kevmoo]

SGTM – although deferring to @mit-mit

[mit-mit]

I'm a bit confused about the timeline as the Mitigation section mentions both 3.1 and 3.2. Can you clarify @brianquinlan ?

[itsjustkevin]

@vsmenon @Hixie and @grouma another breaking change for your review.

[grouma]

As far as I can tell this doesn't impact ACX or our customers. Can we run a global presubmit to validate?

[brianquinlan]

I'm a bit confused about the timeline as the Mitigation section mentions both 3.1 and 3.2. Can you clarify @brianquinlan ?

I was hoping to deprecate the Platform constructor in Dart 3.1
I was hoping to make Platform abstract in Dart 3.2

But @timsneath suggested that Dart 3.2 is too aggressive so we can push that out to... when? I don't know what our policy is here.

[mit-mit]

I suggest a few more releases in between deprecation and the following change.

[vsmenon]

lgtm - let's go forward with marking deprecated

If we're conservative, we'd give a year. We can decide whether we want to pull that forward.

[itsjustkevin]

@vsmenon I would like to mark this as approved, but am still waiting for feedback from @Hixie and some assurance through a global presubmit for @grouma.