Add support for testing with MS Edge

As far as I can tell (from [this list](https://github.com/dart-lang/test/tree/master/lib/src/runner/browser)), there's currently no support for testing in MS Edge. If so, I think it would be useful to add.

Start by reviewing the browser runners listed in lib/src/runner/browser and compare how existing browser support is tested. Determine the scope needed for MS Edge support and identify the relevant test coverage before making changes. Done means the project can run its browser tests against MS Edge with matching coverage to the existing runners.

Written by the indexing model from the issue text.