Use regular functions and not arrow functions in the index.js file of the modules.

[vladimiry]

https://github.com/ethereum/web3.js/blob/5023c450a783b892da862712e0d8dc26cddba4ab/packages/web3-eth/src/index.js#L76

https://github.com/ethereum/web3.js/blob/5023c450a783b892da862712e0d8dc26cddba4ab/packages/web3-eth/src/index.js#L87

In a clean not-transpiled environment such code will give you TransactionSigner is not a constructor error as arrow function cannot be acting as a class constructor::

class EthTransactionSigner {}

const TransactionSigner = () => {
    return new EthTransactionSigner();
};

new TransactionSigner()

It's currently only working probably because Babel transpiles arrow functions to a regular/ES5 functions which can be acting as class constructors.

I'd suggest renaming TransactionSigner and MethodFactory to something like constructTransactionSigner and constructMethodFactory functions and then calling them without using new but as a regular function calls.

[nivida]

First I had such factory methods in each index.js but I've removed them because it would break the API for the standalone modules. This will get improved with the implementation of the Web3 Namespace.

Edit:
I'm using the same pattern also for internal classes to have a consistent way over all index.js files and yes it wouldn't work without babel.

[vladimiry]

Maybe then for the sake of explicitness simply using regular/ES5 functions as they can act as classes.

[nivida]

Yes, right I will improve this later. Currently, there are issues with a higher priority. :)

[nivida]

Changed this issue label to bug because it does not work with the arrow methods if you use a submodule module.
Thanks for figuring this out!