Allow Pubspec.yaml to supply missing SDK libraries (dart:io, dart:html, etc.)

[terrier989]

It would be helpful if something like this was possible:

dependency_overrides:
  "dart:io": universal_io

This would enable use packages like universal_io and universal_html without forking dependencies just to replace imports.

Proposed details:

• The dependency override would be ignored if the target already supports dart:L.
• The effect would be equal to replacing imports of dart:L with package:P/L.dart.
• Behavior of configured imports would not be affected. Condition in if (dart.library.io) 'library.dart' would false in Dart2js even if dart:io was supplied with a dependency override.

[devoncarew]

I'm not sure where to best file this feature request - tentatively assigning to area-library.

[terrier989]

This feature could provide at least temporary help to Flutter web team, which seems to be thinking about cross-platform issues such as missing dart:io (#35969) and dart:developer (#36649). Angular would get server-side rendering in Dart VM (#225).

The proposal could be simplified by using prefix "__" for supplying SDK libraries ("__io", "__html", etc.). This feature would be just a preprocessor in the Dart frontend. In pseudo code:

when the preprocessor sees "import/export dart:L":
  if target SDK doesn't have library "dart:L" and Pub has added "__L:file://path" in ".packages" file:
    replace "dart:L" with "package:__L/L.dart"

With "__libraryName" convention, Pub doesn't need changes. Pub already generates ".packages" file when pubspec.yaml is:

dependency_overrides:
  # This supplies dart:io to Dart2js
  __io:
    git: "https://github.com/some/repository" # Package name must be "__io"

You wouldn't be able to use hosted packages (Pub would throw "package names must be equal"), but it can be fixed separately from this feature.