Explanation of why no tests ran

An excerpt of email by @chalin (emphasis mine):

> pub serve test --port=8080
> Loading source assets...
> Loading angular2, angular2/transform/reflection_remover, test/pub_serve and dart_to_js_script_rewriter transformers...
> Pub "serve" started on http://localhost:8080
> Serving angular_tour_of_heroes test on http://localhost:8080
> Finished compilation. Running tests...
> pub run test --pub-serve=8080
> **No tests ran.**

As I assume it is never the intention of the author to use `pub run test` to run no tests, can test recover and explain why no tests were run? In this particular case it was because all tests were filtered out since the default platform is VM, and all of Patrice's tests were browser based.

I'd like to see:

> No tests ran: The default platform "vm" was used, but tests expected: ["browser"].
>
> See goo.gl/xxx for how to configure the test runner for different platforms.

Start by reproducing `pub run test --pub-serve=8080` with tests filtered out by the default VM platform, then read the test runner's no-tests reporting and platform-selection handling. Done means an empty run explains the selected platform and expected platforms, with guidance for configuring the runner.

Written by the indexing model from the issue text.