Implicitly import "import.dart"

It would be great if `import.dart` or `.import.dart` at project level is implicitly imported in every files of the project so we don't have to manually import commonly used imports.
It would also make easier to use extensions.

Start by reviewing the proposal in this issue alongside the Dart language specification repository context. Determine the intended project-level scope, precedence, and behavior for implicit imports and extensions. Done means the language change is sufficiently specified for review, including its semantics and any compatibility implications.

Written by the indexing model from the issue text.