(Fully) support package:test in DartPad

[lrhn]

The test package is not a supported package. Or rather, the Analyzer thinks it's not, giving the error:

Unsupported package: 'package:test'.

plus the info

The imported package 'test' isn't a dependency of the importing package.

The import actually works if you run the test. The stack trace on the failure looks a little weird, but it's there.

It's sometimes useful to use DartPad to share (possibly failing) tests.
It takes some work to extract the failing test from the test-framework, and that would be much easier of DartPad would allow import 'package:test/test.dart';.

(Even more test integration would be nice too, but simply being able to run is a great start.)