Description

Currently, on plugin-legacy, the legacy polyfill is generated by compilation to ES5.
The problem is that when using "additional polyfill" property for supplying external polyfills, some of them aren't fully legacy compatible.
For example, I get in my project that Chrome 39 doesn't know the keyword "const", so the polyfill loading breaks JS on this browser.

Suggested solution

(Add an option to) transform the additional legacy polyfills by babel preset plugin or something like that, in plugin-legacy.

Alternative

No response

Additional context

Clarification - what I want to be done is already down with the regular JS content, that is being transformed by babel. I want an option to "do the same" for the generated legacy polyfill.

Validations

• Follow our Code of Conduct
• Read the Contributing Guidelines.
• Read the docs.
• Check that there isn't already an issue that request the same feature to avoid creating a duplicate.