Conditional compilation API via @define

[theseanl]

@define in Closure compiler is like environment variables in other build tools. It is a common practice for webpack or rollup to access environment variables in source code (process.env.NODE_ENV), so that the build tool replaces such expressions to a constant, and later a javascript minifier removes unvisited code blocks.
In closure compiler, such a functionality is achieved via @define JsDoc annotation. Can we provide a uniform API for conditional compilation that uses @define at closure compiler’s side and some custom plugin for rollup’s side?

Currently there are some caveats of using @define in goog.module scopes, angular/tsickle#434. Since we control closure compiler flags as well, I think it would be possible to implement an easy API.

[theseanl]

google/closure-compiler#1601 (comment)

1. We could wait for the issue to be fixed (closure compiler implementing @defines without closure library)
2. or we could rewrite @defines to goog.define with ts transformer.