Add support for objects created with a null prototype in objectOf

__tests__/PropTypesDevelopmentStandalone-test.js

@@ -723,6 +723,12 @@ describe('PropTypesDevelopmentStandalone', () => {
       });
     });
 
+    it('should support objects with a null prototype', () => {
+      const nullObj = Object.create(null);
+      nullObj.test = "a property";
+      typeCheckPass(PropTypes.objectOf(PropTypes.string), nullObj);
+    });
+
     it('should warn with invalid items in the object', () => {
       typeCheckFail(
         PropTypes.objectOf(PropTypes.number),

factoryWithTypeCheckers.js

@@ -12,6 +12,7 @@ var assign = require('object-assign');
 var ReactPropTypesSecret = require('./lib/ReactPropTypesSecret');
 var checkPropTypes = require('./checkPropTypes');
 
+var has = Function.call.bind(Object.prototype.hasOwnProperty);
 var printWarning = function() {};
 
 if (process.env.NODE_ENV !== 'production') {
@@ -318,7 +319,7 @@ module.exports = function(isValidElement, throwOnDirectAccess) {
         return new PropTypeError('Invalid ' + location + ' `' + propFullName + '` of type ' + ('`' + propType + '` supplied to `' + componentName + '`, expected an object.'));
       }
       for (var key in propValue) {
-        if (propValue.hasOwnProperty(key)) {
+        if (has(propValue, key)) {
           var error = typeChecker(propValue, key, componentName, location, propFullName + '.' + key, ReactPropTypesSecret);
           if (error instanceof Error) {
             return error;