Custom reporter

[dkrutskikh]

Added ability to define user custom reporter

[googlebot]

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[googlebot]

CLAs look good, thanks!

[kevmoo]

Please revert this file change. .idea files should be ignored in your global .gitignore file

[dkrutskikh]

@kevmoo done

[natebosch]

I'll let @nex3 comment on the change overall

[natebosch]

Avoid methods named setSomething. You can have a set method at the top level, but since this isn't doing any extra work we can also make the variable public instead of private.

[dkrutskikh]

@natebosch done

[nex3]

This approach won't work, for a couple reasons:

• It only works when running tests via dart path/to/test.dart. This is a semi-unofficial way of invoking the test runner, so it doesn't make sense to add features that only work there.

• It exposes the private internal Engine and Reporter APIs. Even when Provide a programmatic API #48 is done, I don't want to make these packages part of the API for the test package itself because additional API surface makes it harder to make large changes without breaking the world.