Add test for stack trace mapping in the browser console

[jakemac53]

This has broken multiple times, so we need to set up a web driver test or something along those lines to check the actual browser console output.

We should also check the package:test output in the terminal once that is actually working (won't require webdriver at least). (this is done)

[matanlurey]

I don't mind trying this today/tomorrow assuming nobody was going to start earlier.

[matanlurey]

Some questions:

• Do we want to verify pub run test maps or just in the browser?
• Do we care about DDC and Dart2JS or just DDC?

[jakemac53]

We do already have a test for the stack traces printed by the test command (but only for DDC).

Ideally we would check both dart2js and ddc though.