DART uses the Winston library for logging. You can find the location of your DART logs by choosing Help > About from the main menu. You should see a dialog like this:
As you develop, you can tail the logs to see what's happening in real time. Just substitute the proper log location in the command below.
tail -f /Users/apd4n/Library/Logs/DART/dart.log
Also note that you can tail the logs directly from DART by choose Help > Logs from the menu. While this approach is handy, there are two caveats. First, if the log file is large, the log window may take a long time to load. Second, DART rotates its logs when they reach a size of 5 MB. If that rollover happens while you're tailing the log in the DART log window, you won't see any entries logged after the rollover.
DART logs to a different file when it runs unit tests, usually to a directory called
.dart-test/log/dart.log. For more information on logging, see the code in util/logger.js and the config settings in core/config.js.