printWarning = function (format) {
    for (var _len = arguments.length, args = Array(_len > 1 ? _len - 1 : 0), _key = 1; _key < _len; _key++) {
      args[_key - 1] = arguments[_key];
    }

    var argIndex = 0;
    var message = 'Warning: ' + format.replace(/%s/g, function () {
      return args[argIndex++];
    });
    if (typeof console !== 'undefined') {
      console.warn(message);
    }
    try {
      // --- Welcome to debugging React ---
      // This error was thrown as a convenience so that you can use this stack
      // to find the callsite that caused this warning to fire.
      throw new Error(message);
    } catch (x) {}
  }
Forked from fbjs/warning:
https://github.com/facebook/fbjs/blob/e66ba20ad5be433eb54423f2b097d829324d9de6/packages/fbjs/src/__forks__/warning.js

Only change is we use console.warn instead of console.error,
and do nothing when 'console' is not supported.
This really simplifies the code.
---
Similar to invariant but only logs a warning if the condition is not met.
This can be used to log issues in development environments in critical
paths. Removing the logging code for production environments will keep the
same logic and follow the same code paths.