Use redefining helper pattern for better tree-shakeability

Author: Andarist

src/compiler/transformers/es2015.ts

@@ -4074,9 +4074,13 @@ namespace ts {
         priority: 0,
         text: `
             var __extends = (this && this.__extends) || (function () {
-                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]; };
+                function extendStatics(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);
+                }
+
                 return function (d, b) {
                     extendStatics(d, b);
                     function __() { this.constructor = d; }

src/compiler/transformers/esnext.ts

@@ -878,13 +878,16 @@ namespace ts {
         scoped: false,
         priority: 1,
         text: `
-            var __assign = (this && this.__assign) || Object.assign || function(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;
+            var __assign = (this && this.__assign) || function () {
+                __assign = Object.assign || function(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 __assign.apply(this, arguments);
             };`
     };