Change isPlainObject to support objects created in a different context

packages/styled-components/src/utils/isPlainObject.js

@@ -1,2 +1,8 @@
 // @flow
-export default (x: any): boolean => typeof x === 'object' && x.constructor === Object;
+import { typeOf } from 'react-is';
+
+export default (x: any): boolean =>
+  x !== null &&
+  typeof x === 'object' &&
+  (x.toString ? x.toString() : Object.prototype.toString.call(x)) === '[object Object]' &&
+  !typeOf(x);

packages/styled-components/src/utils/test/isPlainObject.test.js

@@ -0,0 +1,36 @@
+// @flow
+import vm from "vm";
+import React from "react";
+import isPlainObject from '../isPlainObject';
+
+it('returns true for an object literal', () => {
+  expect(isPlainObject({})).toEqual(true);
+});
+
+it('returns false for an instance of a class with its own toString method', () => {
+  class SomeClass {
+    toString() {
+      return 'some: thing;';
+    }
+  }
+  expect(isPlainObject(new SomeClass())).toEqual(false);
+});
+
+it('returns false for a function', () => {
+  expect(isPlainObject(() => {})).toEqual(false);
+});
+
+it('returns false for an array', () => {
+  expect(isPlainObject([])).toEqual(false);
+});
+
+it('returns false for a React component', () => {
+  class Foo extends React.Component {};
+  expect(isPlainObject(Foo)).toEqual(false);
+});
+
+it('returns true for an object literal created in a different context', () => {
+  const context = vm.createContext({});
+  vm.runInContext('object = {};', context);
+  expect(isPlainObject(context.object)).toEqual(true);
+});