Support debugging VM tests

[nex3]

The test runner should make it very easy to run an interactive debugger for VM tests, whether it be via the command line, observatory, or an IDE. This is blocked on issue 23320, which will allow the test runner to enable the observatory server at runtime.

[kevmoo]

@nex3 FYI: update this issue with description RE observatory

[kevmoo]

Also would like/need ability to start an isolate in checked mode - https://code.google.com/p/dart/issues/detail?id=21791

[johnpryan]

sdk issue 23320 is marked as 'not planned'. Could we get some clarification on what that means exactly?

It's still possible to debug tests by creating an HTML file with <script> tags and debug in WebStorm, but that file needs to be maintained, and you cant use the -n flag or solo_ to filter tests.

[kevmoo]

See my comments here: dart-lang/pub#1018 (comment)

Short version: we'll make this work