Fix object spread according to spec (#7034)

packages/babel-plugin-proposal-object-rest-spread/test/fixtures/object-spread/expression/input.js

@@ -1 +1,7 @@
 ({ x, ...y, a, ...b, c });
+
+({ ...Object.prototype });
+
+({ ...{ foo: 'bar' } });
+
+({ ...{ get foo () { return 'foo' } } });

packages/babel-plugin-proposal-object-rest-spread/test/fixtures/object-spread/expression/output.js

@@ -1,9 +1,24 @@
-function _extends() { _extends = Object.assign || function (target) { for (var i = 1; i < arguments.length; i++) { var source = arguments[i]; for (var key in source) { if (Object.prototype.hasOwnProperty.call(source, key)) { target[key] = source[key]; } } } return target; }; return _extends.apply(this, arguments); }
+function _objectSpread(target) { for (var i = 1; i < arguments.length; i++) { var source = arguments[i] != null ? arguments[i] : {}; var ownKeys = Object.keys(source); if (typeof Object.getOwnPropertySymbols === 'function') { ownKeys = ownKeys.concat(Object.getOwnPropertySymbols(source).filter(function (sym) { return Object.getOwnPropertyDescriptor(source, sym).enumerable; })); } ownKeys.forEach(function (key) { _defineProperty(target, key, source[key]); }); } return target; }
 
-_extends({
+function _defineProperty(obj, key, value) { if (key in obj) { Object.defineProperty(obj, key, { value: value, enumerable: true, configurable: true, writable: true }); } else { obj[key] = value; } return obj; }
+
+_objectSpread({
   x
 }, y, {
   a
 }, b, {
   c
 });
+
+_objectSpread({}, Object.prototype);
+
+_objectSpread({}, {
+  foo: 'bar'
+});
+
+_objectSpread({}, {
+  get foo() {
+    return 'foo';
+  }
+
+});