Make VM prefer .dart_tool over .packages.

[mkustermann]

The VM will start to support .dart_tool/package_config.json for resolving package Uris in various scenarios.

We do have a Isolate.packageConfig API in dart:isolate which says atm:

  /**
   * The package root of the current isolate, if any.
   *
   * If the isolate is using a [packageRoot] (**) or the isolate has not been
   * setup for package resolution, this getter returns `null`, otherwise it
   * returns the package config URI.
   *
   * (**) the deprecated packages/ folder
   */
  external static Future<Uri> get packageConfig;

Now the question is: Should this getter return a Uri to .dart_tool/package_config.json if the VM is using it for package resolution? Or should we expose a new get packageJsonConfig getter?

Right now there are a few packages out there, e.g. package:package_resolver which invoke this getter and expect the file at that Uri to be of .packages format (and not the json format). This package is in fact used by e.g. flutter atm (see /src/flutter/lib/web_ui/dev/test_platform.dart)

Those packages will break once the VM favors .dart_tool/package_config.json over .packages.

[lrhn]

Should this getter return a Uri to .dart_tool/package_config.json if the VM is using it for package resolution?

Yes.

There won't be any .packages file to refer to soon enough. Anyone needing to understand the file contents will need to understand the new file format.

[mkustermann]

Thanks for the confirmation, @lrhn

I have filed flutter/flutter#56289 for the flutter team to take this on.

Will re-classify this issue as area-vm and mark it blocked until flutter/flutter#56289 has been addressed.

[mkustermann]

Flutter has moved to Lasse's new package, so we should be unblocked on preferring to use the new .dart_tool/package_config.json over .packages.

Made CL: https://dart-review.googlesource.com/c/sdk/+/146840