Silly mocha+babel problem with relative imports outside project root

submoduiles.js

How do you mocha+babel test projectA as is? 
Note that projectB also its own mocha+babel tests which run fine (but they do not need to run here).

// ..../somedir/projectA/src/index.js
import {x} from '../../projectB/src/index.js';
...


// ..../somedir/projectB/src/index.js
import {x} from './tools.js';
...


// something like this works for all my projects, including projectB
node_modules/.bin/mocha --compilers js:babel-core/register tests/specs/**/*.js

How do I make it work for projectA without blowing up on the import keyword 
in projectB? Note that I explicitly want to import projectB as es6.  

The solution was to make sure a .babelrc exists in all folders included this way. One was missing which blew up the stack.