Make sure define(obj, key, value) always works, even in IE8.

packages/regenerator-runtime/runtime.js

@@ -8,7 +8,6 @@
 var runtime = (function (exports) {
   "use strict";
 
-  var define = Object.defineProperty;
   var Op = Object.prototype;
   var hasOwn = Op.hasOwnProperty;
   var undefined; // More compressible than void 0.
@@ -17,13 +16,22 @@ var runtime = (function (exports) {
   var asyncIteratorSymbol = $Symbol.asyncIterator || "@@asyncIterator";
   var toStringTagSymbol = $Symbol.toStringTag || "@@toStringTag";
 
-  if (define) {
-    try {
-      // IE 8 has a broken Object.defineProperty that only works on DOM objects
-      define({}, '', {});
-    } catch (err) {
-      define = undefined;
-    }
+  function define(obj, key, value) {
+    Object.defineProperty(obj, key, {
+      value: value,
+      enumerable: true,
+      configurable: true,
+      writable: true
+    });
+    return obj[key];
+  }
+  try {
+    // IE 8 has a broken Object.defineProperty that only works on DOM objects.
+    define({}, "");
+  } catch (err) {
+    define = function(obj, key, value) {
+      return obj[key] = value;
+    };
   }
 
   function wrap(innerFn, outerFn, self, tryLocsList) {
@@ -92,35 +100,23 @@ var runtime = (function (exports) {
     IteratorPrototype = NativeIteratorPrototype;
   }
 
-  function ensureDefaultToStringTag(object, defaultValue) {
-    // https://bugzilla.mozilla.org/show_bug.cgi?id=1644581#c6
-    return toStringTagSymbol in object
-      ? (define && define(object, toStringTagSymbol, {
-          value: defaultValue,
-          enumerable: true,
-          configurable: true,
-          writable: true
-        }),
-        defaultValue)
-      : object[toStringTagSymbol] = defaultValue;
-  }
-
   var Gp = GeneratorFunctionPrototype.prototype =
     Generator.prototype = Object.create(IteratorPrototype);
   GeneratorFunction.prototype = Gp.constructor = GeneratorFunctionPrototype;
   GeneratorFunctionPrototype.constructor = GeneratorFunction;
-  GeneratorFunction.displayName = ensureDefaultToStringTag(
+  GeneratorFunction.displayName = define(
     GeneratorFunctionPrototype,
+    toStringTagSymbol,
     "GeneratorFunction"
   );
 
   // Helper for defining the .next, .throw, and .return methods of the
   // Iterator interface in terms of a single ._invoke method.
   function defineIteratorMethods(prototype) {
     ["next", "throw", "return"].forEach(function(method) {
-      prototype[method] = function(arg) {
+      define(prototype, method, function(arg) {
         return this._invoke(method, arg);
-      };
+      });
     });
   }
 
@@ -139,7 +135,7 @@ var runtime = (function (exports) {
       Object.setPrototypeOf(genFun, GeneratorFunctionPrototype);
     } else {
       genFun.__proto__ = GeneratorFunctionPrototype;
-      ensureDefaultToStringTag(genFun, "GeneratorFunction");
+      define(genFun, toStringTagSymbol, "GeneratorFunction");
     }
     genFun.prototype = Object.create(Gp);
     return genFun;
@@ -409,7 +405,7 @@ var runtime = (function (exports) {
   // unified ._invoke helper method.
   defineIteratorMethods(Gp);
 
-  ensureDefaultToStringTag(Gp, "Generator");
+  define(Gp, toStringTagSymbol, "Generator");
 
   // A Generator should always return itself as the iterator object when the
   // @@iterator function is called on it. Some browsers' implementations of the