Expected behavior

plugin's transform function returns {code, map}

Actual behavior

transform function returns only code like {code}

and the console warns with messages:

Sourcemap is likely to be incorrect: a plugin ('vue.delegate') was used to transform files, but didn't generate a sourcemap for the transformation. Consult the plugin documentation for help

Steps to reproduce the behavior

when update to v4，my project's source map could not work，it is caused by the dependency module vue-component-compiler's assembler that only return {code}.