Small improvements to et lint command

[johnmccutchan]

• Default to dumping out lint logs (can be disabled with --quiet flag).
• Add Logger.fatal which logs an error and throws a FatalError which is caught in main.
• Simplify findDartBinDirectory implementation.
• Make JSON serialized process artifacts more human readable.

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[johnmccutchan]

lgtm modulo test failures. I think it's possible to write unit tests of some of this as well, like the fatal() log message, at least.

added test here.