diff --git a/packages/babel-helpers/src/helpers.js b/packages/babel-helpers/src/helpers.js
index a83613b674..f05e23bcc4 100644
--- a/packages/babel-helpers/src/helpers.js
+++ b/packages/babel-helpers/src/helpers.js
@@ -474,10 +474,12 @@ helpers.inherits = helper("7.0.0-beta.0")`
 `;
 
 helpers.inheritsLoose = helper("7.0.0-beta.0")`
+  import setPrototypeOf from "setPrototypeOf";
+
   export default function _inheritsLoose(subClass, superClass) {
     subClass.prototype = Object.create(superClass.prototype);
     subClass.prototype.constructor = subClass;
-    subClass.__proto__ = superClass;
+    setPrototypeOf(subClass, superClass);
   }
 `;
 
diff --git a/packages/babel-plugin-transform-classes/test/fixtures/extend-builtins/loose/output.js b/packages/babel-plugin-transform-classes/test/fixtures/extend-builtins/loose/output.js
index a52a5f4da9..bbdc4b88e5 100644
--- a/packages/babel-plugin-transform-classes/test/fixtures/extend-builtins/loose/output.js
+++ b/packages/babel-plugin-transform-classes/test/fixtures/extend-builtins/loose/output.js
@@ -1,4 +1,4 @@
-function _inheritsLoose(subClass, superClass) { subClass.prototype = Object.create(superClass.prototype); subClass.prototype.constructor = subClass; subClass.__proto__ = superClass; }
+function _inheritsLoose(subClass, superClass) { subClass.prototype = Object.create(superClass.prototype); subClass.prototype.constructor = subClass; _setPrototypeOf(subClass, superClass); }
 
 function _wrapNativeSuper(Class) { var _cache = typeof Map === "function" ? new Map() : undefined; _wrapNativeSuper = function _wrapNativeSuper(Class) { if (Class === null || !_isNativeFunction(Class)) return Class; if (typeof Class !== "function") { throw new TypeError("Super expression must either be null or a function"); } if (typeof _cache !== "undefined") { if (_cache.has(Class)) return _cache.get(Class); _cache.set(Class, Wrapper); } function Wrapper() { return _construct(Class, arguments, _getPrototypeOf(this).constructor); } Wrapper.prototype = Object.create(Class.prototype, { constructor: { value: Wrapper, enumerable: false, writable: true, configurable: true } }); return _setPrototypeOf(Wrapper, Class); }; return _wrapNativeSuper(Class); }
 
diff --git a/packages/babel-plugin-transform-classes/test/fixtures/get-set-loose/call-semantics-data-defined-on-parent/output.js b/packages/babel-plugin-transform-classes/test/fixtures/get-set-loose/call-semantics-data-defined-on-parent/output.js
index 36ae19da78..4c0bcc52e1 100644
--- a/packages/babel-plugin-transform-classes/test/fixtures/get-set-loose/call-semantics-data-defined-on-parent/output.js
+++ b/packages/babel-plugin-transform-classes/test/fixtures/get-set-loose/call-semantics-data-defined-on-parent/output.js
@@ -1,6 +1,8 @@
 "use strict";
 
-function _inheritsLoose(subClass, superClass) { subClass.prototype = Object.create(superClass.prototype); subClass.prototype.constructor = subClass; subClass.__proto__ = superClass; }
+function _inheritsLoose(subClass, superClass) { subClass.prototype = Object.create(superClass.prototype); subClass.prototype.constructor = subClass; _setPrototypeOf(subClass, superClass); }
+
+function _setPrototypeOf(o, p) { _setPrototypeOf = Object.setPrototypeOf || function _setPrototypeOf(o, p) { o.__proto__ = p; return o; }; return _setPrototypeOf(o, p); }
 
 let Base = /*#__PURE__*/function () {
   function Base() {}
diff --git a/packages/babel-plugin-transform-classes/test/fixtures/get-set-loose/call-semantics-getter-defined-on-parent/output.js b/packages/babel-plugin-transform-classes/test/fixtures/get-set-loose/call-semantics-getter-defined-on-parent/output.js
index 22e33e1fbd..da4d0029d0 100644
--- a/packages/babel-plugin-transform-classes/test/fixtures/get-set-loose/call-semantics-getter-defined-on-parent/output.js
+++ b/packages/babel-plugin-transform-classes/test/fixtures/get-set-loose/call-semantics-getter-defined-on-parent/output.js
@@ -1,6 +1,8 @@
 "use strict";
 
-function _inheritsLoose(subClass, superClass) { subClass.prototype = Object.create(superClass.prototype); subClass.prototype.constructor = subClass; subClass.__proto__ = superClass; }
+function _inheritsLoose(subClass, superClass) { subClass.prototype = Object.create(superClass.prototype); subClass.prototype.constructor = subClass; _setPrototypeOf(subClass, superClass); }
+
+function _setPrototypeOf(o, p) { _setPrototypeOf = Object.setPrototypeOf || function _setPrototypeOf(o, p) { o.__proto__ = p; return o; }; return _setPrototypeOf(o, p); }
 
 function _defineProperties(target, props) { for (var i = 0; i < props.length; i++) { var descriptor = props[i]; descriptor.enumerable = descriptor.enumerable || false; descriptor.configurable = true; if ("value" in descriptor) descriptor.writable = true; Object.defineProperty(target, descriptor.key, descriptor); } }
 
diff --git a/packages/babel-plugin-transform-classes/test/fixtures/get-set-loose/call-semantics-not-defined-on-parent/output.js b/packages/babel-plugin-transform-classes/test/fixtures/get-set-loose/call-semantics-not-defined-on-parent/output.js
index 35437a744d..1e76b8c863 100644
--- a/packages/babel-plugin-transform-classes/test/fixtures/get-set-loose/call-semantics-not-defined-on-parent/output.js
+++ b/packages/babel-plugin-transform-classes/test/fixtures/get-set-loose/call-semantics-not-defined-on-parent/output.js
@@ -1,6 +1,8 @@
 "use strict";
 
-function _inheritsLoose(subClass, superClass) { subClass.prototype = Object.create(superClass.prototype); subClass.prototype.constructor = subClass; subClass.__proto__ = superClass; }
+function _inheritsLoose(subClass, superClass) { subClass.prototype = Object.create(superClass.prototype); subClass.prototype.constructor = subClass; _setPrototypeOf(subClass, superClass); }
+
+function _setPrototypeOf(o, p) { _setPrototypeOf = Object.setPrototypeOf || function _setPrototypeOf(o, p) { o.__proto__ = p; return o; }; return _setPrototypeOf(o, p); }
 
 let Base = function Base() {};
 
diff --git a/packages/babel-plugin-transform-classes/test/fixtures/get-set-loose/call-semantics-setter-defined-on-parent/output.js b/packages/babel-plugin-transform-classes/test/fixtures/get-set-loose/call-semantics-setter-defined-on-parent/output.js
index 43755b886d..d0d3d9071d 100644
--- a/packages/babel-plugin-transform-classes/test/fixtures/get-set-loose/call-semantics-setter-defined-on-parent/output.js
+++ b/packages/babel-plugin-transform-classes/test/fixtures/get-set-loose/call-semantics-setter-defined-on-parent/output.js
@@ -1,6 +1,8 @@
 "use strict";
 
-function _inheritsLoose(subClass, superClass) { subClass.prototype = Object.create(superClass.prototype); subClass.prototype.constructor = subClass; subClass.__proto__ = superClass; }
+function _inheritsLoose(subClass, superClass) { subClass.prototype = Object.create(superClass.prototype); subClass.prototype.constructor = subClass; _setPrototypeOf(subClass, superClass); }
+
+function _setPrototypeOf(o, p) { _setPrototypeOf = Object.setPrototypeOf || function _setPrototypeOf(o, p) { o.__proto__ = p; return o; }; return _setPrototypeOf(o, p); }
 
 function _defineProperties(target, props) { for (var i = 0; i < props.length; i++) { var descriptor = props[i]; descriptor.enumerable = descriptor.enumerable || false; descriptor.configurable = true; if ("value" in descriptor) descriptor.writable = true; Object.defineProperty(target, descriptor.key, descriptor); } }
 
diff --git a/packages/babel-plugin-transform-classes/test/fixtures/get-set-loose/get-semantics-data-defined-on-parent/output.js b/packages/babel-plugin-transform-classes/test/fixtures/get-set-loose/get-semantics-data-defined-on-parent/output.js
index 073dbb8f90..e3abdfc5b2 100644
--- a/packages/babel-plugin-transform-classes/test/fixtures/get-set-loose/get-semantics-data-defined-on-parent/output.js
+++ b/packages/babel-plugin-transform-classes/test/fixtures/get-set-loose/get-semantics-data-defined-on-parent/output.js
@@ -1,6 +1,8 @@
 "use strict";
 
-function _inheritsLoose(subClass, superClass) { subClass.prototype = Object.create(superClass.prototype); subClass.prototype.constructor = subClass; subClass.__proto__ = superClass; }
+function _inheritsLoose(subClass, superClass) { subClass.prototype = Object.create(superClass.prototype); subClass.prototype.constructor = subClass; _setPrototypeOf(subClass, superClass); }
+
+function _setPrototypeOf(o, p) { _setPrototypeOf = Object.setPrototypeOf || function _setPrototypeOf(o, p) { o.__proto__ = p; return o; }; return _setPrototypeOf(o, p); }
 
 let Base = function Base() {};
 
diff --git a/packages/babel-plugin-transform-classes/test/fixtures/get-set-loose/get-semantics-getter-defined-on-parent/output.js b/packages/babel-plugin-transform-classes/test/fixtures/get-set-loose/get-semantics-getter-defined-on-parent/output.js
index e100018292..21cf51afd7 100644
--- a/packages/babel-plugin-transform-classes/test/fixtures/get-set-loose/get-semantics-getter-defined-on-parent/output.js
+++ b/packages/babel-plugin-transform-classes/test/fixtures/get-set-loose/get-semantics-getter-defined-on-parent/output.js
@@ -1,6 +1,8 @@
 "use strict";
 
-function _inheritsLoose(subClass, superClass) { subClass.prototype = Object.create(superClass.prototype); subClass.prototype.constructor = subClass; subClass.__proto__ = superClass; }
+function _inheritsLoose(subClass, superClass) { subClass.prototype = Object.create(superClass.prototype); subClass.prototype.constructor = subClass; _setPrototypeOf(subClass, superClass); }
+
+function _setPrototypeOf(o, p) { _setPrototypeOf = Object.setPrototypeOf || function _setPrototypeOf(o, p) { o.__proto__ = p; return o; }; return _setPrototypeOf(o, p); }
 
 function _defineProperties(target, props) { for (var i = 0; i < props.length; i++) { var descriptor = props[i]; descriptor.enumerable = descriptor.enumerable || false; descriptor.configurable = true; if ("value" in descriptor) descriptor.writable = true; Object.defineProperty(target, descriptor.key, descriptor); } }
 
diff --git a/packages/babel-plugin-transform-classes/test/fixtures/get-set-loose/get-semantics-not-defined-on-parent/output.js b/packages/babel-plugin-transform-classes/test/fixtures/get-set-loose/get-semantics-not-defined-on-parent/output.js
index 62247a8c4d..e4c7eb3249 100644
--- a/packages/babel-plugin-transform-classes/test/fixtures/get-set-loose/get-semantics-not-defined-on-parent/output.js
+++ b/packages/babel-plugin-transform-classes/test/fixtures/get-set-loose/get-semantics-not-defined-on-parent/output.js
@@ -1,6 +1,8 @@
 "use strict";
 
-function _inheritsLoose(subClass, superClass) { subClass.prototype = Object.create(superClass.prototype); subClass.prototype.constructor = subClass; subClass.__proto__ = superClass; }
+function _inheritsLoose(subClass, superClass) { subClass.prototype = Object.create(superClass.prototype); subClass.prototype.constructor = subClass; _setPrototypeOf(subClass, superClass); }
+
+function _setPrototypeOf(o, p) { _setPrototypeOf = Object.setPrototypeOf || function _setPrototypeOf(o, p) { o.__proto__ = p; return o; }; return _setPrototypeOf(o, p); }
 
 let Base = function Base() {};
 
diff --git a/packages/babel-plugin-transform-classes/test/fixtures/get-set-loose/get-semantics-setter-defined-on-parent/output.js b/packages/babel-plugin-transform-classes/test/fixtures/get-set-loose/get-semantics-setter-defined-on-parent/output.js
index 9afd6ef654..d1e5164378 100644
--- a/packages/babel-plugin-transform-classes/test/fixtures/get-set-loose/get-semantics-setter-defined-on-parent/output.js
+++ b/packages/babel-plugin-transform-classes/test/fixtures/get-set-loose/get-semantics-setter-defined-on-parent/output.js
@@ -1,6 +1,8 @@
 "use strict";
 
-function _inheritsLoose(subClass, superClass) { subClass.prototype = Object.create(superClass.prototype); subClass.prototype.constructor = subClass; subClass.__proto__ = superClass; }
+function _inheritsLoose(subClass, superClass) { subClass.prototype = Object.create(superClass.prototype); subClass.prototype.constructor = subClass; _setPrototypeOf(subClass, superClass); }
+
+function _setPrototypeOf(o, p) { _setPrototypeOf = Object.setPrototypeOf || function _setPrototypeOf(o, p) { o.__proto__ = p; return o; }; return _setPrototypeOf(o, p); }
 
 function _defineProperties(target, props) { for (var i = 0; i < props.length; i++) { var descriptor = props[i]; descriptor.enumerable = descriptor.enumerable || false; descriptor.configurable = true; if ("value" in descriptor) descriptor.writable = true; Object.defineProperty(target, descriptor.key, descriptor); } }
 
diff --git a/packages/babel-plugin-transform-classes/test/fixtures/get-set-loose/memoized-assign/output.js b/packages/babel-plugin-transform-classes/test/fixtures/get-set-loose/memoized-assign/output.js
index 92cb4f6d39..cffc803238 100644
--- a/packages/babel-plugin-transform-classes/test/fixtures/get-set-loose/memoized-assign/output.js
+++ b/packages/babel-plugin-transform-classes/test/fixtures/get-set-loose/memoized-assign/output.js
@@ -1,6 +1,8 @@
 "use strict";
 
-function _inheritsLoose(subClass, superClass) { subClass.prototype = Object.create(superClass.prototype); subClass.prototype.constructor = subClass; subClass.__proto__ = superClass; }
+function _inheritsLoose(subClass, superClass) { subClass.prototype = Object.create(superClass.prototype); subClass.prototype.constructor = subClass; _setPrototypeOf(subClass, superClass); }
+
+function _setPrototypeOf(o, p) { _setPrototypeOf = Object.setPrototypeOf || function _setPrototypeOf(o, p) { o.__proto__ = p; return o; }; return _setPrototypeOf(o, p); }
 
 let Base = function Base() {};
 
diff --git a/packages/babel-plugin-transform-classes/test/fixtures/get-set-loose/memoized-update/output.js b/packages/babel-plugin-transform-classes/test/fixtures/get-set-loose/memoized-update/output.js
index 950af23a42..4565f4fa54 100644
--- a/packages/babel-plugin-transform-classes/test/fixtures/get-set-loose/memoized-update/output.js
+++ b/packages/babel-plugin-transform-classes/test/fixtures/get-set-loose/memoized-update/output.js
@@ -1,6 +1,8 @@
 "use strict";
 
-function _inheritsLoose(subClass, superClass) { subClass.prototype = Object.create(superClass.prototype); subClass.prototype.constructor = subClass; subClass.__proto__ = superClass; }
+function _inheritsLoose(subClass, superClass) { subClass.prototype = Object.create(superClass.prototype); subClass.prototype.constructor = subClass; _setPrototypeOf(subClass, superClass); }
+
+function _setPrototypeOf(o, p) { _setPrototypeOf = Object.setPrototypeOf || function _setPrototypeOf(o, p) { o.__proto__ = p; return o; }; return _setPrototypeOf(o, p); }
 
 let Base = function Base() {};
 
diff --git a/packages/babel-plugin-transform-classes/test/fixtures/get-set-loose/set-semantics-data-defined-on-parent/output.js b/packages/babel-plugin-transform-classes/test/fixtures/get-set-loose/set-semantics-data-defined-on-parent/output.js
index bfb30c8524..ed6bd9d7a4 100644
--- a/packages/babel-plugin-transform-classes/test/fixtures/get-set-loose/set-semantics-data-defined-on-parent/output.js
+++ b/packages/babel-plugin-transform-classes/test/fixtures/get-set-loose/set-semantics-data-defined-on-parent/output.js
@@ -1,6 +1,8 @@
 "use strict";
 
-function _inheritsLoose(subClass, superClass) { subClass.prototype = Object.create(superClass.prototype); subClass.prototype.constructor = subClass; subClass.__proto__ = superClass; }
+function _inheritsLoose(subClass, superClass) { subClass.prototype = Object.create(superClass.prototype); subClass.prototype.constructor = subClass; _setPrototypeOf(subClass, superClass); }
+
+function _setPrototypeOf(o, p) { _setPrototypeOf = Object.setPrototypeOf || function _setPrototypeOf(o, p) { o.__proto__ = p; return o; }; return _setPrototypeOf(o, p); }
 
 let Base = function Base() {};
 
diff --git a/packages/babel-plugin-transform-classes/test/fixtures/get-set-loose/set-semantics-getter-defined-on-parent/output.js b/packages/babel-plugin-transform-classes/test/fixtures/get-set-loose/set-semantics-getter-defined-on-parent/output.js
index 386a31be80..d4812193a7 100644
--- a/packages/babel-plugin-transform-classes/test/fixtures/get-set-loose/set-semantics-getter-defined-on-parent/output.js
+++ b/packages/babel-plugin-transform-classes/test/fixtures/get-set-loose/set-semantics-getter-defined-on-parent/output.js
@@ -1,6 +1,8 @@
 "use strict";
 
-function _inheritsLoose(subClass, superClass) { subClass.prototype = Object.create(superClass.prototype); subClass.prototype.constructor = subClass; subClass.__proto__ = superClass; }
+function _inheritsLoose(subClass, superClass) { subClass.prototype = Object.create(superClass.prototype); subClass.prototype.constructor = subClass; _setPrototypeOf(subClass, superClass); }
+
+function _setPrototypeOf(o, p) { _setPrototypeOf = Object.setPrototypeOf || function _setPrototypeOf(o, p) { o.__proto__ = p; return o; }; return _setPrototypeOf(o, p); }
 
 function _defineProperties(target, props) { for (var i = 0; i < props.length; i++) { var descriptor = props[i]; descriptor.enumerable = descriptor.enumerable || false; descriptor.configurable = true; if ("value" in descriptor) descriptor.writable = true; Object.defineProperty(target, descriptor.key, descriptor); } }
 
diff --git a/packages/babel-plugin-transform-classes/test/fixtures/get-set-loose/set-semantics-not-defined-on-parent-data-on-obj/output.js b/packages/babel-plugin-transform-classes/test/fixtures/get-set-loose/set-semantics-not-defined-on-parent-data-on-obj/output.js
index 6bee588327..cbbae75137 100644
--- a/packages/babel-plugin-transform-classes/test/fixtures/get-set-loose/set-semantics-not-defined-on-parent-data-on-obj/output.js
+++ b/packages/babel-plugin-transform-classes/test/fixtures/get-set-loose/set-semantics-not-defined-on-parent-data-on-obj/output.js
@@ -1,6 +1,8 @@
 "use strict";
 
-function _inheritsLoose(subClass, superClass) { subClass.prototype = Object.create(superClass.prototype); subClass.prototype.constructor = subClass; subClass.__proto__ = superClass; }
+function _inheritsLoose(subClass, superClass) { subClass.prototype = Object.create(superClass.prototype); subClass.prototype.constructor = subClass; _setPrototypeOf(subClass, superClass); }
+
+function _setPrototypeOf(o, p) { _setPrototypeOf = Object.setPrototypeOf || function _setPrototypeOf(o, p) { o.__proto__ = p; return o; }; return _setPrototypeOf(o, p); }
 
 let Base = function Base() {};
 
diff --git a/packages/babel-plugin-transform-classes/test/fixtures/get-set-loose/set-semantics-not-defined-on-parent-getter-on-obj/output.js b/packages/babel-plugin-transform-classes/test/fixtures/get-set-loose/set-semantics-not-defined-on-parent-getter-on-obj/output.js
index 80aa04f8eb..f68a10f545 100644
--- a/packages/babel-plugin-transform-classes/test/fixtures/get-set-loose/set-semantics-not-defined-on-parent-getter-on-obj/output.js
+++ b/packages/babel-plugin-transform-classes/test/fixtures/get-set-loose/set-semantics-not-defined-on-parent-getter-on-obj/output.js
@@ -4,7 +4,9 @@ function _defineProperties(target, props) { for (var i = 0; i < props.length; i+
 
 function _createClass(Constructor, protoProps, staticProps) { if (protoProps) _defineProperties(Constructor.prototype, protoProps); if (staticProps) _defineProperties(Constructor, staticProps); return Constructor; }
 
-function _inheritsLoose(subClass, superClass) { subClass.prototype = Object.create(superClass.prototype); subClass.prototype.constructor = subClass; subClass.__proto__ = superClass; }
+function _inheritsLoose(subClass, superClass) { subClass.prototype = Object.create(superClass.prototype); subClass.prototype.constructor = subClass; _setPrototypeOf(subClass, superClass); }
+
+function _setPrototypeOf(o, p) { _setPrototypeOf = Object.setPrototypeOf || function _setPrototypeOf(o, p) { o.__proto__ = p; return o; }; return _setPrototypeOf(o, p); }
 
 let Base = function Base() {};
 
diff --git a/packages/babel-plugin-transform-classes/test/fixtures/get-set-loose/set-semantics-not-defined-on-parent-not-on-obj/output.js b/packages/babel-plugin-transform-classes/test/fixtures/get-set-loose/set-semantics-not-defined-on-parent-not-on-obj/output.js
index 6035452d69..a5c75b5081 100644
--- a/packages/babel-plugin-transform-classes/test/fixtures/get-set-loose/set-semantics-not-defined-on-parent-not-on-obj/output.js
+++ b/packages/babel-plugin-transform-classes/test/fixtures/get-set-loose/set-semantics-not-defined-on-parent-not-on-obj/output.js
@@ -1,6 +1,8 @@
 "use strict";
 
-function _inheritsLoose(subClass, superClass) { subClass.prototype = Object.create(superClass.prototype); subClass.prototype.constructor = subClass; subClass.__proto__ = superClass; }
+function _inheritsLoose(subClass, superClass) { subClass.prototype = Object.create(superClass.prototype); subClass.prototype.constructor = subClass; _setPrototypeOf(subClass, superClass); }
+
+function _setPrototypeOf(o, p) { _setPrototypeOf = Object.setPrototypeOf || function _setPrototypeOf(o, p) { o.__proto__ = p; return o; }; return _setPrototypeOf(o, p); }
 
 let Base = function Base() {};
 
diff --git a/packages/babel-plugin-transform-classes/test/fixtures/get-set-loose/set-semantics-not-defined-on-parent-setter-on-obj/output.js b/packages/babel-plugin-transform-classes/test/fixtures/get-set-loose/set-semantics-not-defined-on-parent-setter-on-obj/output.js
index d3349f28f1..61d6f91876 100644
--- a/packages/babel-plugin-transform-classes/test/fixtures/get-set-loose/set-semantics-not-defined-on-parent-setter-on-obj/output.js
+++ b/packages/babel-plugin-transform-classes/test/fixtures/get-set-loose/set-semantics-not-defined-on-parent-setter-on-obj/output.js
@@ -4,7 +4,9 @@ function _defineProperties(target, props) { for (var i = 0; i < props.length; i+
 
 function _createClass(Constructor, protoProps, staticProps) { if (protoProps) _defineProperties(Constructor.prototype, protoProps); if (staticProps) _defineProperties(Constructor, staticProps); return Constructor; }
 
-function _inheritsLoose(subClass, superClass) { subClass.prototype = Object.create(superClass.prototype); subClass.prototype.constructor = subClass; subClass.__proto__ = superClass; }
+function _inheritsLoose(subClass, superClass) { subClass.prototype = Object.create(superClass.prototype); subClass.prototype.constructor = subClass; _setPrototypeOf(subClass, superClass); }
+
+function _setPrototypeOf(o, p) { _setPrototypeOf = Object.setPrototypeOf || function _setPrototypeOf(o, p) { o.__proto__ = p; return o; }; return _setPrototypeOf(o, p); }
 
 let Base = function Base() {};
 
diff --git a/packages/babel-plugin-transform-classes/test/fixtures/get-set-loose/set-semantics-setter-defined-on-parent/output.js b/packages/babel-plugin-transform-classes/test/fixtures/get-set-loose/set-semantics-setter-defined-on-parent/output.js
index 85df344609..a0304c9b26 100644
--- a/packages/babel-plugin-transform-classes/test/fixtures/get-set-loose/set-semantics-setter-defined-on-parent/output.js
+++ b/packages/babel-plugin-transform-classes/test/fixtures/get-set-loose/set-semantics-setter-defined-on-parent/output.js
@@ -1,6 +1,8 @@
 "use strict";
 
-function _inheritsLoose(subClass, superClass) { subClass.prototype = Object.create(superClass.prototype); subClass.prototype.constructor = subClass; subClass.__proto__ = superClass; }
+function _inheritsLoose(subClass, superClass) { subClass.prototype = Object.create(superClass.prototype); subClass.prototype.constructor = subClass; _setPrototypeOf(subClass, superClass); }
+
+function _setPrototypeOf(o, p) { _setPrototypeOf = Object.setPrototypeOf || function _setPrototypeOf(o, p) { o.__proto__ = p; return o; }; return _setPrototypeOf(o, p); }
 
 function _defineProperties(target, props) { for (var i = 0; i < props.length; i++) { var descriptor = props[i]; descriptor.enumerable = descriptor.enumerable || false; descriptor.configurable = true; if ("value" in descriptor) descriptor.writable = true; Object.defineProperty(target, descriptor.key, descriptor); } }
 
diff --git a/packages/babel-plugin-transform-classes/test/fixtures/loose-classCallCheck/with-superClass/output.js b/packages/babel-plugin-transform-classes/test/fixtures/loose-classCallCheck/with-superClass/output.js
index d2dd7f34e2..5295d0ece0 100644
--- a/packages/babel-plugin-transform-classes/test/fixtures/loose-classCallCheck/with-superClass/output.js
+++ b/packages/babel-plugin-transform-classes/test/fixtures/loose-classCallCheck/with-superClass/output.js
@@ -1,6 +1,8 @@
 function _assertThisInitialized(self) { if (self === void 0) { throw new ReferenceError("this hasn't been initialised - super() hasn't been called"); } return self; }
 
-function _inheritsLoose(subClass, superClass) { subClass.prototype = Object.create(superClass.prototype); subClass.prototype.constructor = subClass; subClass.__proto__ = superClass; }
+function _inheritsLoose(subClass, superClass) { subClass.prototype = Object.create(superClass.prototype); subClass.prototype.constructor = subClass; _setPrototypeOf(subClass, superClass); }
+
+function _setPrototypeOf(o, p) { _setPrototypeOf = Object.setPrototypeOf || function _setPrototypeOf(o, p) { o.__proto__ = p; return o; }; return _setPrototypeOf(o, p); }
 
 let B = function B() {
   "use strict";