It doesn’t have a `@flow` annotation. We should start Flow-ifying it to avoid bugs like https://github.com/facebook/react/pull/11173.

@iamdustan Wanna take it?
var contextTypes = type.contextTypes;
  if (!contextTypes) {
    return emptyObject;
  }
  var maskedContext = {};
  for (var contextName in contextTypes) {
    maskedContext[contextName] = context[contextName];
  }
  return maskedContext;
}
function checkContextTypes(typeSpecs, values, location: string) {