Do not warn about externals when using transform runtime.

[swernerx]

For bundling our library we are using "transform-runtime" together with polyfill: false flag. Even if setting runtimeHelpers: true as suggested in the documentation we are seeing this confusing message:

The '${helper}' Babel helper is used more than once in your code. 

which should probably not happen in my opinion. Looking through the code it seems changing this in https://github.com/rollup/rollup-plugin-babel/blob/master/src/index.js#L64 should be enough:

} else {

into

} else (helpers !== RUNTIME) {

[swernerx]

BTW: We have one bundle were we do not transpile es2015 classes (for modern clients/nodejs). In this case the whole detection in preflightCheck.js might be broken as it explicitely check for usage of the class helper to detect the runtime usage.

[Rich-Harris]

Fixed in 2.7.1 – thanks