Update helpers to use redefining pattern if needed (related to micros…

…oft/TypeScript#24244) (#54)

tslib.es6.js

@@ -14,22 +14,28 @@ and limitations under the License.
 ***************************************************************************** */
 /* global Reflect, Promise */
 
-var extendStatics = Object.setPrototypeOf ||
-    ({ __proto__: [] } instanceof Array && function (d, b) { d.__proto__ = b; }) ||
-    function (d, b) { for (var p in b) if (b.hasOwnProperty(p)) d[p] = b[p]; };
+var extendStatics = function(d, b) {
+    extendStatics = Object.setPrototypeOf ||
+        ({ __proto__: [] } instanceof Array && function (d, b) { d.__proto__ = b; }) ||
+        function (d, b) { for (var p in b) if (b.hasOwnProperty(p)) d[p] = b[p]; };
+    return extendStatics(d, b);
+};
 
 export function __extends(d, b) {
     extendStatics(d, b);
     function __() { this.constructor = d; }
     d.prototype = b === null ? Object.create(b) : (__.prototype = b.prototype, new __());
 }
 
-export var __assign = Object.assign || function __assign(t) {
-    for (var s, i = 1, n = arguments.length; i < n; i++) {
-        s = arguments[i];
-        for (var p in s) if (Object.prototype.hasOwnProperty.call(s, p)) t[p] = s[p];
+export var __assign = function() {
+    __assign = Object.assign || function __assign(t) {
+        for (var s, i = 1, n = arguments.length; i < n; i++) {
+            s = arguments[i];
+            for (var p in s) if (Object.prototype.hasOwnProperty.call(s, p)) t[p] = s[p];
+        }
+        return t;
     }
-    return t;
+    return __assign.apply(this, arguments);
 }
 
 export function __rest(s, e) {