https://twitter.com/therealyashsriv/status/1219691914523545601

We shouldn't generate code that might cause browser or linting to complain.

https://github.com/facebook/react/blob/0cf22a56a18790ef34c71bef14f64695c0498619/packages/legacy-events/SyntheticEvent.js#L259

It's also just a confusing pattern at best.
this.isDefaultPrevented = functionThatReturnsTrue;
  } else {
    this.isDefaultPrevented = functionThatReturnsFalse;
  }
  this.isPropagationStopped = functionThatReturnsFalse;
  return this;
}
Object.assign(SyntheticEvent.prototype, {
  preventDefault: function() {
    this.defaultPrevented = true;
    const event = this.nativeEvent;
    if (!event) {
      return;
    }
    if (event.preventDefault) {
      event.preventDefault();
    } else if (typeof event.returnValue !== 'unknown') {
      event.returnValue = false;
    }
    this.isDefaultPrevented = functionThatReturnsTrue;
  },
  stopPropagation: function() {
    const event = this.nativeEvent;
    if (!event) {
      return;
    }
    if (event.stopPropagation) {
      event.stopPropagation();
    } else if (typeof event.cancelBubble !== 'unknown') {