badImport.js

export const functionToImport = () => {
  // I had an old reference to a class method that was in this file previously
  // React never consold an error but I saw the warning `Did not expect componentDidMount timer to start while render timer is still in progress for another instance`
  // luckily I had few very changes in my working directory, so I didn't spend too much time trying to find the culprit
  var x = this.something(); 
  
  return x;
}

fileImporting.js

import React, {
    PropTypes
} from 'react';

import {
    Text
} from 'react-native';

import { functionToImport } from './badImport.js';

var MyComponent = () => {
    // you must call the imported function for the warning to fire!
    
    return (
        <Text>{ functionToImport() }</Text>
    )
};