Add support for dart2_constant

[nex3]

The dart2_constant package provides camel-case constant definitions that work on both Dart 1 and Dart 2, for packages that want to support both. It would be great if this package could add support for migrating to uses of these polyfill constants in addition to using the Dart 2 constants directly.

[devoncarew]

Ah, migrating from package:dart2_constant's utf8 to dart:convert's utf8?

[nex3]

That, but also migrating from Dart 1's UTF8 to package:dart2_constant's convert.utf8.

[devoncarew]

Thanks for the suggestion; I think I'm going to mark this as out of scope for this package.