To reproduce:

• Clone
• npm install
• npm run watch
• npm run test-only

The error is:

/.../babylon/lib/tokenizer/types.js:47
  babelHelpers.inherits(KeywordTokenType, _TokenType);
  ^

ReferenceError: babelHelpers is not defined
    at C:\Users\anhans\work\bbl\babylon\lib\tokenizer\types.js:47:3

The error only happens when using watch. If you use npm run build instead, there's no error.
Removing the "external-helpers" plugin from .babelrc fixes the problem, but I presume it's there for a reason.