optimize __createBinding (#168)

Reflect microsoft/TypeScript#46997: When the binding is itself one that was created by `__createBinding`, re-use its descriptor, which avoids piling multiple levels of getters in the case of multiple levels of exports. In addition, reuse a descriptor if the bindings is marked as non-writable and non-configurable, which makes a getter not necessary. (This can be done manually if needed, even though tsc doesn't do it now.) Could be considered as a fix for #165 -- first, this PR prevents piling up multiple layers of getters. Second, it allows a hack of adding if (typeof exports === "object") exports = Object.freeze(exports); to avoid getters altogether. (And in the future, tsc could mark `const` exports as non-writable and non-configurable which would make it possible to avoid this hack.)

Author: elibarzilay

tslib.es6.js

@@ -107,7 +107,11 @@ export function __generator(thisArg, body) {
 
 export var __createBinding = Object.create ? (function(o, m, k, k2) {
  if (k2 === undefined) k2 = k;
- Object.defineProperty(o, k2, { enumerable: true, get: function() { return m[k]; } });
+ var desc = Object.getOwnPropertyDescriptor(m, k);
+ if (!desc || ("get" in desc ? !m.__esModule : desc.writable || desc.configurable)) {
+ desc = { enumerable: true, get: function() { return m[k]; } };
+ }
+ Object.defineProperty(o, k2, desc);
 }) : (function(o, m, k, k2) {
  if (k2 === undefined) k2 = k;
  o[k2] = m[k];

tslib.js

@@ -153,7 +153,11 @@ var __createBinding;
 
  __createBinding = Object.create ? (function(o, m, k, k2) {
  if (k2 === undefined) k2 = k;
- Object.defineProperty(o, k2, { enumerable: true, get: function() { return m[k]; } });
+ var desc = Object.getOwnPropertyDescriptor(m, k);
+ if (!desc || ("get" in desc ? !m.__esModule : desc.writable || desc.configurable)) {
+ desc = { enumerable: true, get: function() { return m[k]; } };
+ }
+ Object.defineProperty(o, k2, desc);
  }) : (function(o, m, k, k2) {
  if (k2 === undefined) k2 = k;
  o[k2] = m[k];