Do not print duplicate error messages to stdout.

[yifanyang]

The issue with regard to lerna duplicating error messages was resolved recently. With this lerna option '--stream', lerna will output the same error message only once.

See the comparison below. In below demonstrations, I manually added a case that is meant to fail in firestore sdk test:

1. error message without this change: code change, travis logs
2. error message with this change: code change, traivs logs

Please check how the same error output are duplicated 4 times in the previous way:

• messages printed by mocha;
• messages printed by lerna (prefixed with "lerna ERR!");
• messages printed by lerna (in the "Error" filed of the error object at the end of the log);
• messages printed by lerna (in the "stdout" field of the error object at the end of the log).

[mikelehen]

Sounds good to me, but I'll defer to Fei to officially approve. Thanks!

[yifanyang]

Sure. Thanks! I invited both of you since I think this should be helpful to firestore most, as firestore generates much more (usually thousands of) lines of log than other sdks. The duplication would cause a big confusion in such case.

[mikelehen]

Yeah, I just want to defer to Fei since upgrading lerna from 2.x to 3.x could have other impacts on the repo and he's the person to sign off on that. :-)

[Feiyang1]

Lerna 3 doesn't have any breaking change that affects us, so it should be fine.

[yifanyang]

Tested both with and without the --stream option. Looks like the explicit --stream is necessary for lerna to not duplicate logs.