DISCUSS: Should the default output language be `NO_TRANSPILE`?

It is now possible to output ES_2017 code, and in future we would really prefer to stay more on top of the output language level.

Currently the default output language level is still ES5 to support the most possible browsers by default. Would it now make sense to make the default NO_TRANSPILE, so you get the same language level out as you get in unless you specify otherwise?

Note that we’re still almost certain to support new language levels as input before they are supported for output (just with a much smaller gap between). In such cases NO_TRANSPILE wouldn’t work and we’d need to generate an error if the output language weren’t explicitly specified.

Or would we default to the highest possible? That seems dangerous, since a new release of the compiler that silently switches the output from, say, ES_2019 to ES_2020 would break projects that were unintentionally relying on the fact that the default was ES_2019 just because we hadn’t yet added support for ES_2020

[ctjlewis]

@customcommander Use --compilation_level=WHITESPACE_ONLY for that use case. NO_TRANSPILE is a language_out flag.

[brad4d]

@christiantjl If NO_TRANSPILE were the default, then projects using the default would end up broken when we add new features that are understood only by the front end (checks) of the compiler and not the back end (optimizations) yet.

Also, it would mean that the default output level would change a lot more frequently, likely taking people unpleasantly by surprise.